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/config.el

@@ -0,0 +1,92 @@
+;;; completion/corfu/config.el -*- lexical-binding: t; -*-
+
+(defvar +corfu-want-ret-to-confirm t
+  "Configure how the user expects RET to behave.
+Possible values are:
+- t (default): Insert candidate if one is selected, pass-through otherwise;
+- `minibuffer': Insert candidate if one is selected, pass-through otherwise,
+              and immediatelly exit if in the minibuffer;
+- nil: Pass-through without inserting.")
+
+;;
+;;; Packages
+(use-package! corfu
+  :hook (doom-first-input . global-corfu-mode)
+  :init
+  (add-hook! 'minibuffer-setup-hook
+    (defun +corfu-enable-in-minibuffer ()
+      "Enable Corfu in the minibuffer if `completion-at-point' is bound."
+      (when (where-is-internal #'completion-at-point (list (current-local-map)))
+        (setq-local corfu-echo-delay nil)
+        (corfu-mode +1))))
+  :config
+  (setq corfu-auto t
+        corfu-auto-delay 0.1
+        corfu-auto-prefix 2
+        global-corfu-modes '((not
+                              erc-mode
+                              circe-mode
+                              help-mode
+                              gud-mode
+                              vterm-mode)
+                             t)
+        corfu-cycle t
+        corfu-separator (when (modulep! +orderless) ?\s)
+        corfu-preselect 'prompt
+        corfu-count 16
+        corfu-max-width 120
+        corfu-preview-current 'insert
+        corfu-on-exact-match nil
+        corfu-quit-at-boundary (if (modulep! +orderless) 'separator t)
+        corfu-quit-no-match (if (modulep! +orderless) 'separator t)
+        tab-always-indent 'complete)
+  (add-to-list 'completion-category-overrides `(lsp-capf (styles ,@completion-styles)))
+  (add-to-list 'corfu-auto-commands #'lispy-colon)
+  (add-to-list 'corfu-continue-commands #'+corfu-move-to-minibuffer)
+  (add-hook 'evil-insert-state-exit-hook #'corfu-quit))
+
+(use-package! cape
+  :defer t
+  :init
+  (add-hook! prog-mode
+    (defun +corfu-add-cape-file-h ()
+      (add-to-list 'completion-at-point-functions #'cape-file)))
+  (add-hook! (org-mode markdown-mode)
+    (defun +corfu-add-cape-elisp-block-h ()
+      (add-hook 'completion-at-point-functions #'cape-elisp-block 0 t)))
+
+  ;; Make these capfs composable.
+  (advice-add #'lsp-completion-at-point :around #'cape-wrap-noninterruptible)
+  (advice-add #'lsp-completion-at-point :around #'cape-wrap-nonexclusive)
+  (advice-add #'comint-completion-at-point :around #'cape-wrap-nonexclusive)
+  (advice-add #'eglot-completion-at-point :around #'cape-wrap-nonexclusive)
+  (advice-add #'pcomplete-completions-at-point :around #'cape-wrap-nonexclusive)
+  ;; From the `cape' readme. Without this, Eshell autocompletion is broken on
+  ;; Emacs28.
+  (when (< emacs-major-version 29)
+    (advice-add 'pcomplete-completions-at-point :around #'cape-wrap-silent)
+    (advice-add 'pcomplete-completions-at-point :around #'cape-wrap-purify)))
+
+(use-package! corfu-terminal
+  :when (not (display-graphic-p))
+  :hook ((corfu-mode . corfu-terminal-mode)))
+
+;;
+;;; Extensions
+
+(use-package! corfu-history
+  :hook ((corfu-mode . corfu-history-mode))
+  :config
+  (after! savehist (add-to-list 'savehist-additional-variables 'corfu-history)))
+
+(use-package! corfu-popupinfo
+  :hook ((corfu-mode . corfu-popupinfo-mode))
+  :config
+  (setq corfu-popupinfo-delay '(0.5 . 1.0)))
+
+(use-package! nerd-icons-corfu
+  :when (modulep! +icons)
+  :defer t
+  :init
+  (after! corfu
+    (add-to-list 'corfu-margin-formatters #'nerd-icons-corfu-formatter)))