feat(completion): add support for GitHub Copilot via copilot.el

I opted for accepting copilot completions using C-<tab> /
<backtab> (shift+tab) instead of regular tab, as it was often getting in
the way of yasnippet and lsp completion suggestions. This allows a more
explicit acceptance of Copilot suggestions.

core/siren-core-modules.el

@@ -19,8 +19,9 @@
 (require 'siren-packages)
 
 ;; Completion
-(require 'siren-company)
 (require 'siren-cape)
+(require 'siren-company)
+(require 'siren-copilot)
 
 ;; Documentation
 (require 'siren-dash-at-point)

modules/completion/siren-cape.el

@@ -9,6 +9,7 @@
 (require 'siren-company)
 
 (use-package cape
+  :after company
   :hook
   (lsp-completion-mode . siren-cape-capf-lsp-mode-setup)
 

modules/completion/siren-copilot.el

@@ -0,0 +1,28 @@
+;;; siren-copilot.el --- jimeh's Emacs Siren: copilot configuration.
+
+;;; Commentary:
+
+;; Basic configuration for copilot. Auto completion on steroids.
+
+;;; Code:
+
+(use-package copilot
+  :straight (:host github :repo "zerolfx/copilot.el"
+                   :files ("dist" "copilot.el"))
+  :hook
+  (prog-mode . copilot-mode)
+
+  :general
+  ("C-<tab>" 'copilot-accept-completion)
+  ("<backtab>" 'copilot-accept-completion)
+  (:keymaps 'company-active-map
+            "C-<tab>" 'copilot-accept-completion
+            "C-TAB" 'copilot-accept-completion
+            "<backtab>" 'copilot-accept-completion)
+
+  :config
+  (with-eval-after-load 'company
+    (delq 'company-preview-if-just-one-frontend company-frontends)))
+
+(provide 'siren-copilot)
+;;; siren-copilot.el ends here