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`.

crates/zed/src/zed.rs

@@ -2690,7 +2690,7 @@ mod tests {
         theme::init(theme::LoadThemes::JustBase, cx);
 
         let mut has_default_theme = false;
-        for theme_name in themes.list(false).map(|meta| meta.name) {
+        for theme_name in themes.list(false).into_iter().map(|meta| meta.name) {
             let theme = themes.get(&theme_name).unwrap();
             assert_eq!(theme.name, theme_name);
             if theme.name == ThemeSettings::get(None, cx).active_theme.name {