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;

templates/init.example.el

@@ -22,6 +22,7 @@
 
        :completion
        company           ; the ultimate code completion backend
+       ;;(corfu +orderless)  ; complete with cap(f), cape and a flying feather!
        ;;helm              ; the *other* search engine for love and life
        ;;ido               ; the other *other* search engine...
        ;;ivy               ; a search engine for love and life