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/tools/lsp/+lsp.el

@@ -138,8 +138,11 @@ server getting expensively restarted when reverting buffers."
                                    " "))
           (add-to-list 'global-mode-string
                        '(t (:eval lsp-modeline-icon))
-                       'append))))))
+                       'append)))))
 
+  (when (modulep! :completion corfu)
+    (setq lsp-completion-provider :none)
+    (add-hook 'lsp-mode-hook #'lsp-completion-mode)))
 
 (use-package! lsp-ui
   :hook (lsp-mode . lsp-ui-mode)