feat(tree-sitter): add +tree-sitter/doctor command

For a quick diagnosis of the state of tree-sitter support in the current
buffer.

modules/tools/tree-sitter/README.org

@@ -75,6 +75,9 @@ requires two additional steps:
    version constraints; newer versions of some grammars may not work with older
    versions of Emacs).
 
+To test whether tree-sitter is functioning in the current buffer, execute ~M-x
++tree-sitter/doctor~.
+
 * TODO Usage
 #+begin_quote
  󱌣 /This module's usage documentation is incomplete./ [[doom-contrib-module:][Complete it?]]

modules/tools/tree-sitter/autoload/tree-sitter.el

@@ -69,6 +69,14 @@ pre-Emacs 31."
   (let (auto-mode-alist interpreter-mode-alist)
     (apply fn args)))
 
-;;; TODO: Backwards compatibility
+;;;###autoload
+(defun +tree-sitter/doctor ()
+  "Test if the current buffer is correctly tree-sitter-enabled."
+  (interactive)
+  (unless (treesit-available-p)
+    (user-error "This Emacs install wasn't built with treesit support!"))
+  (unless (treesit-parser-list)
+    (user-error "No tree-sitter parsers are active in this buffer; are the grammars properly installed?"))
+  (message "Tree-sitter is functioning in this buffer!"))
 
 ;;; tree-sitter.el ends here