Add support for loading user themes (#7027)

This PR adds support for loading user themes in Zed.

Themes are loaded from the `themes` directory under the Zed config:
`~/.config/zed/themes`. This directory should contain JSON files
containing a `ThemeFamilyContent`.

Here's an example of the general structure of a theme family file:

```jsonc
{
  "name": "Vitesse",
  "author": "Anthony Fu",
  "themes": [
    {
      "name": "Vitesse Dark Soft",
      "appearance": "dark",
      "style": {
        "border": "#252525",
        // ...
      }
    }
  ]
}
```

Themes placed in this directory will be loaded and available in the
theme selector.

Release Notes:

- Added support for loading user themes from `~/.config/zed/themes`.