selectrum: add icon support

Use the package `all-the-icons-completion` to add icons to vertical
completion buffers.

modules/completion/selectrum/config.el

@@ -158,6 +158,9 @@
 (use-package! marginalia
   :hook (doom-first-input . marginalia-mode)
   :init
+  (when (featurep! +icons)
+    (add-hook 'marginalia-mode-hook
+              (lambda () (all-the-icons-completion-mode (if marginalia-mode 1 -1)))))
   (map! :map minibuffer-local-map
         :desc "Cycle marginalia views"
         "M-A" #'marginalia-cycle)