This commit's primary goal is allowing use of
[minad/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
[minad/cape](https://github.com/minad/cape) capfs for certain modes.
Other optional or miscellaneous features include:
- Support for displaying the completion's documentation on a secondary
popup;
- Support for terminal display if :os tty;
- Support for icons if +icons;
- Support for tab-and-go completion if +tng;
