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/completion/corfu/autoload.el

@@ -0,0 +1,10 @@
+;;; completion/corfu/autoload.el -*- lexical-binding: t; -*-
+
+;;;###autoload
+(defun +corfu-move-to-minibuffer ()
+  ;; Taken from corfu's README.
+  ;; TODO: extend this to other completion front-ends.
+  (interactive)
+  (let ((completion-extra-properties corfu--extra)
+        (completion-cycle-threshold completion-cycling))
+    (apply #'consult-completion-in-region completion-in-region--data)))