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.