💥 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/lang/ocaml/README.org

@@ -54,7 +54,7 @@ opam install merlin utop ocp-indent dune ocamlformat
   on-the-fly syntax/type checking via =merlin=, otherwise this is only done when
   the file is saved.
 + spell checking is activated in comments if =:tools flyspell= is active
-+ a REPL is provided if =utop= is installed and =:feature eval= is active
++ a REPL is provided if =utop= is installed and =:tools eval= is active
 + if =:editor format= is enabled, the =ocamlformat= executable is available and
   there is an =.ocamlformat= file present then =format-all-buffer= is bound to
   =ocamlformat=, otherwise to =ocp-indent=