💥 Remove :feature category

:feature was a "catch-all" category. Many of its modules fit better in
other categories, so they've been moved:

- feature/debugger -> tools/debugger
- feature/evil -> editor/evil
- feature/eval -> tools/eval
- feature/lookup -> tools/lookup
- feature/snippets -> editor/snippets
- feature/file-templates -> editor/file-templates
- feature/workspaces -> ui/workspaces

More potential changes in the future:

- A new :term category for terminal emulation modules (eshell, term and
  vterm).
- A new :os category for modules dedicated to os-specific functionality.
  The :tools macos module would fit here, but so would modules for nixos
  and arch.
- A new :services category for web-service integration, like wakatime,
  twitter, elfeed, gist and pastebin services.

modules/app/notmuch/autoload.el

@@ -4,8 +4,8 @@
 (defun =notmuch ()
   "Activate (or switch to) `notmuch' in its workspace."
   (interactive)
-  (unless (featurep! :feature workspaces)
-    (user-error ":feature workspaces is required, but disabled"))
+  (unless (featurep! :ui workspaces)
+    (user-error ":ui workspaces is required, but disabled"))
   (condition-case-unless-debug e
       (progn
         (+workspace-switch "*MAIL*" t)