feat(icons): add nonicons.nvim icon provider support (#30)

* feat(icons): add nonicons.nvim icon provider support

Problem: oil.nvim only recognizes mini.icons and nvim-web-devicons as
icon providers. nonicons.nvim works when paired with devicons (via its
apply() monkey-patch), but has no standalone support.

Solution: add a nonicons.nvim fallback in get_icon_provider(), placed
after devicons so the patched devicons path is preferred when both are
installed. The standalone path handles directories via
nonicons.get('file-directory'), files via filetype/extension lookup with
a generic file icon fallback.

* fix(doc): improve readme phrasing

README.md

@@ -21,10 +21,10 @@ https://user-images.githubusercontent.com/506791/209727111-6b4a11f4-634a-4efa-94
 ## Requirements
 
 - Neovim 0.10+
-- Optional:
-  [mini.icons](https://github.com/nvim-mini/mini.nvim/blob/main/readmes/mini-icons.md)
-  or [nvim-web-devicons](https://github.com/nvim-tree/nvim-web-devicons) for
-  file icons
+- (Optionally) any of the following icon providers:
+  - [mini.icons](https://github.com/nvim-mini/mini.nvim/blob/main/readmes/mini-icons.md)
+  - [nvim-web-devicons](https://github.com/nvim-tree/nvim-web-devicons)
+  - [nonicons.nvim](https://github.com/barrettruth/nonicons.nvim)
 
 ## Installation