Add CAPFs from cape:
- `cape-dabbrev`;
- `cape-elisp-block`;
- `cape-file`;
Fix some CAPFs via cape:
- Make non-exclusive, purified and silent `pcomplete-completions-at-point`;
- Make non-exclusive and non-interruptable `lsp-completion-at-point`;
- Make non-exclusive `eglot-completion-at-point`;
- Make non-exclusive `comint-completion-at-point`;
Fix and improve keybindings:
- Smart `DEL`;
Add depth to CAPFs, allowing ordering to be adjustable.
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;
