Refactor file icons to use `IconTheme` (#23153)

This PR adds the initial concept of an `IconTheme` and refactors
`FileIcons` to use it to resolve the icons.

The `IconTheme` will ultimately be used to allow users to select a
different set of icons to use. Currently, however, this is just laying
the foundation for that work.

The association between file types and icons is now handled by the icon
theme when we resolve file icons. This mapping has been moved out of
`file_types.json` and into `icon_theme.rs`.

Release Notes:

- N/A