feat(eww): Add jump-to-heading in EWW

1. Add jump-to-heading functionality for EWW buffers
2. Bind within imenu (replaces +eww/jump-to-url-on-page; bound to
`:localleader l`)

The new functions are based on existing `eww--capture-url-on-page` and
`+eww/jump-to-url-on-page`. However, I took a different approach and
used alists to hide the position/coordinates from the `completing-read`.
Also, unlike `+eww/jump-to-url-on-page`, we don't give the user an
option of limiting the scope to a region or visible portion of the
buffer. `+eww/jump-to-heading-on-page` always prompts based on the
entire buffer.

Examples:

1. `<h1>H1</h1>`
    - "H1"
2. `<h1>H1</h1><h2>H2</h2>`
    - "H1"
    - "H1/h2"
3. `<h1>H1</h1><h2>H2</h2><h3>H3</h3>`
    - "H1"
    - "H1/H2"
    - "H1/H2/H3"
4. `<h1>H1-1</h1><h2>H2</h2><h1>H1-2</h1>`
    - "H1-1"
    - "H1-1/H2"
    - "H1-2"

![screenshot on the Emacs Wikipedia entry](https://github.com/user-attachments/assets/c2210f0f-c026-4325-9b1b-c2427ec13cd5)

Gaps in the hierarchy (for example a `<h2>` followed by an `<h5>`) are not
represented in the labels presented to the user. Take the Wikipedia
entry for Emacs (above) as an example. The `<h2>` "Content" is the first
heading on the page, there's no preceeding `<h1>`, so it's shown to the
user as "Content" without any prefix. Examples:

1. `<h2>H2</h2>`
    - "H2"
2. `<h2>H2</h2><h4>H4</h4>`
    - "H2"
    - "H2/H4"
3. `<h2>H2</h2><h4>H4</h4><h5>H5</h5>`
    - "H2"
    - "H2/H4"
    - "H2/H4/H5"
4. `<h2>H2</h2><h1>H1</h1><h5>H5</h5>`
    - "H2"
    - "H1"
    - "H1/H5"

- modules/emacs/eww/autoload.el
  - (eww--capture-url-on-page): Rename to `eww--capture-urls-on-page`
  - (eww--capture-headings-on-page): Add; based on existing `eww--capture-urls-on-page`
  - (+eww/jump-to-heading-on-page): Add; based on existing `+eww/jump-to-url-on-page`
- modules/emacs/eww/config.el
  - (keybind) Bind `+eww/jump-to-heading-on-page` to `<:localleader.>`;
    based on existing org-mode jump-to-heading keybind (`consult-org-heading`)

modules/emacs/eww/autoload.el

@@ -4,17 +4,15 @@
 ;;   dotfiles. See https://protesilaos.com/codelog/2021-03-25-emacs-eww
 
 ;; Adapted from `prot-eww-jump-to-url-on-page'
-(defun eww--capture-url-on-page (&optional position)
+(defun eww--capture-urls-on-page (&optional position)
   "Capture all the links on the current web page.
 
-Return a list of strings. Strings are in the form LABEL @ URL.
+Return a list of strings. Strings are in the form \"LABEL @ URL\".
 When optional argument POSITION is non-nil, include position info in the strings
-too, so strings take the form: LABEL @ URL ~ POSITION."
+too, so strings take the form: \"POSITION ~ LABEL @ URL\"."
   (let (links match)
     (save-excursion
       (goto-char (point-max))
-      ;; NOTE 2021-07-25: The first clause in the `or' is meant to address a bug
-      ;; where if a URL is in `point-min' it does not get captured.
       (while (setq match (text-property-search-backward 'shr-url))
         (let* ((raw-url (prop-match-value match))
                (start-point-prop (prop-match-beginning match))
@@ -38,6 +36,48 @@ too, so strings take the form: LABEL @ URL ~ POSITION."
                   links)))))
     links))
 
+(defun eww--capture-headings-on-page ()
+  "Return an alist in the form \"LABEL . POINT\" for the current buffer."
+  (let ((heading-stack '())
+        headings match)
+    (save-excursion
+      (goto-char (point-min))
+      (while (setq match (text-property-search-forward 'outline-level))
+        (let* ((level (prop-match-value match))
+               (start-point-prop (prop-match-beginning match))
+               (end-point-prop (prop-match-end match))
+               (text (replace-regexp-in-string
+                      "\n" " "      ; NOTE 2021-07-25: newlines break completion
+                      (buffer-substring-no-properties
+                       start-point-prop end-point-prop))))
+          (cond
+           ((= level (length heading-stack))
+            (pop heading-stack)
+            (push text heading-stack))
+           ((< level (length heading-stack))
+            ;; There's an upward gap between headings (for example: h5, then h2)
+            (dotimes (_ (1+ (- (length heading-stack) level)))
+              (pop heading-stack))
+            (push text heading-stack))
+           ((> level (length heading-stack))
+            ;; There's a downward gap between headings (for example: h2, then h5)
+            (dotimes (_ (1- (- level (length heading-stack))))
+              (push nil heading-stack))
+            (push text heading-stack)))
+          (push (cons
+                 (concat
+                  (let ((preceeding-heading-stack (remove nil (cdr heading-stack))))
+                    (when preceeding-heading-stack
+                      (propertize
+                       (concat
+                        (string-join (reverse preceeding-heading-stack) "/")
+                        "/")
+                       'face 'shadow)))
+                  (car heading-stack))
+                 start-point-prop)
+                headings))))
+    headings))
+
 ;; Adapted from `prot-eww--rename-buffer'
 (defun +eww-page-title-or-url (&rest _)
   (let ((prop (if (string-empty-p (plist-get eww-data :title)) :url :title)))
@@ -75,12 +115,12 @@ consider whole buffer."
     (user-error "Not in an eww buffer!"))
   (let* ((links
           (if arg
-              (eww--capture-url-on-page t)
+              (eww--capture-urls-on-page t)
             (save-restriction
               (if (use-region-p)
                   (narrow-to-region (region-beginning) (region-end))
                 (narrow-to-region (window-start) (window-end)))
-              (eww--capture-url-on-page t))))
+              (eww--capture-urls-on-page t))))
          (prompt-scope (if arg
                            (propertize "URL on the page" 'face 'warning)
                          "visible URL"))
@@ -91,6 +131,18 @@ consider whole buffer."
     (goto-char point)
     (recenter)))
 
+;; Adapted from `prot-eww-jump-to-url-on-page'
+;;;###autoload
+(defun +eww/jump-to-heading-on-page ()
+  "Jump to heading position on the page (whole buffer) using completion."
+  (interactive nil 'eww-mode)
+  (unless (derived-mode-p 'eww-mode)
+    (user-error "Not in an eww buffer!"))
+  (let* ((headings (eww--capture-headings-on-page))
+         (selection (completing-read "Jump to heading: " headings nil t)))
+    (goto-char (alist-get selection headings nil nil #'string=))
+    (recenter)))
+
 ;; Adapted from `prot-eww-open-in-other-window'
 ;;;###autoload
 (defun +eww/open-in-other-window ()

modules/emacs/eww/config.el

@@ -6,7 +6,7 @@
   (map! :map eww-mode-map
         [remap text-scale-increase] #'+eww/increase-font-size
         [remap text-scale-decrease] #'+eww/decrease-font-size
-        [remap imenu] #'+eww/jump-to-url-on-page
+        [remap imenu] #'+eww/jump-to-heading-on-page
         [remap quit-window] #'+eww/quit
         :ni [C-return] #'+eww/open-in-other-window
         :n "yy" #'+eww/copy-current-url
@@ -16,6 +16,7 @@
         (:localleader
          :desc "external browser" "e" #'eww-browse-with-external-browser
          :desc "buffers" "b" #'eww-switch-to-buffer
+         :desc "jump to link" "l" #'+eww/jump-to-url-on-page
 
          (:prefix ("t" . "toggle")
           :desc "readable" "r" #'eww-readable