Allow icon themes to provide their own file associations (#24926)

This PR adds the ability for icon themes to provide their own file
associations.

The old `file_types.json` that was previously used to make these
associations has been removed in favor of storing them on the default
theme.

Icon themes have two new fields on them:

- `file_stems`: A mapping of file stems to icon keys.
- `file_suffixes`: A mapping of file suffixes to icon keys.

These mappings produce icon keys which can then be used in `file_icons`
to associate them to a particular icon:

```json
{
  "file_stems": {
    "Makefile": "make"
  },
  "file_suffixes": {
    "idr": "idris"
  },
  "file_icons": {
    "idris": { "path": "./icons/idris.svg" },
    "make": { "path": "./icons/make.svg" }
  }
}
```

When loading an icon theme, the `file_stems` and `file_icons` fields
will be merged with the ones from the base icon theme, with the values
from the icon theme being loaded overriding ones in the base theme.

Release Notes:

- Added the ability for icon themes to provide their own file
associations.

crates/vim/src/test/vim_test_context.rs

@@ -1,6 +1,5 @@
 use std::ops::{Deref, DerefMut};
 
-use assets::Assets;
 use editor::test::editor_lsp_test_context::EditorLspTestContext;
 use gpui::{Context, Entity, SemanticVersion, UpdateGlobal};
 use search::{project_search::ProjectSearchBar, BufferSearchBar};
@@ -21,7 +20,7 @@ impl VimTestContext {
             cx.set_global(settings);
             release_channel::init(SemanticVersion::default(), cx);
             command_palette::init(cx);
-            project_panel::init(Assets, cx);
+            project_panel::init(cx);
             git_ui::init(cx);
             crate::init(cx);
             search::init(cx);