This tidy ups the corfu module to use the logic when a user enables
corfu +orderless without enabling vertico. Since that's probably a niche
situation, we only specify commit to pin to orderless in the vertico
module so that contributors aren't tripped up with trying to remember to
update orderless in two different code locations. Of course, if this
turns out to be a more popular use-case we can re-evaluate this logic.
Yasnippet is now properly integrated! A previosly-unset default has now
been given to `corfu-on-exact-match`. With snippets, it causes immediate
expansion upon single match by default, so we set it to nil and
recommend against changing it in the README.
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;
