module: add :completion corfu

This commit's primary goal is allowing use of
[Corfu](https://github.com/minad/corfu) as an alternative to
[Company](https://github.com/company-mode/company-mode). It introduces a
module under `:completion` for this purpose, plus some conditionals on
other relevant modules to toggle functionality like lsp back-ends and
[Cape](https://github.com/minad/cape) capfs for certain modes.

Other optional or miscellaneous features include:

- Corfu is enabled in the minibuffer if `completion-at-point` is bound;
- Support for displaying the completion's documentation on a secondary
  popup;
- Support for terminal display if :os tty;
- Support for icons if +icons;

modules/config/default/+emacs-bindings.el

@@ -511,7 +511,7 @@
       "C-x C-b"     #'ibuffer
       "C-x K"       #'doom/kill-this-buffer-in-all-windows
 
-      ;;; company-mode
+      ;;; completion (in-buffer)
       (:when (modulep! :completion company)
        "C-;" #'+company/complete
        (:after company