Add infrastructure for loading icon themes from extensions (#23203)

This PR adds the supporting infrastructure to support loading icon
themes defined by extensions.

Here's an example icon theme:

```json
{
  "name": "My Icon Theme",
  "author": "Me <me@example.com>",
  "themes": [
    {
      "name": "My Icon Theme",
      "appearance": "dark",
      "file_icons": {
        "gleam": { "path": "./icons/file_type_gleam.svg" },
        "toml": { "path": "./icons/file_type_toml.svg" }
      }
    }
  ]
}
```

The icon paths are resolved relative to the root of the extension
directory.

Release Notes:

- N/A

crates/theme_extension/src/theme_extension.rs

@@ -44,4 +44,37 @@ impl ExtensionThemeProxy for ThemeRegistryProxy {
     fn reload_current_theme(&self, cx: &mut AppContext) {
         ThemeSettings::reload_current_theme(cx)
     }
+
+    fn list_icon_theme_names(
+        &self,
+        icon_theme_path: PathBuf,
+        fs: Arc<dyn Fs>,
+    ) -> Task<Result<Vec<String>>> {
+        self.executor.spawn(async move {
+            let icon_theme_family = theme::read_icon_theme(&icon_theme_path, fs).await?;
+            Ok(icon_theme_family
+                .themes
+                .into_iter()
+                .map(|theme| theme.name)
+                .collect())
+        })
+    }
+
+    fn remove_icon_themes(&self, icon_themes: Vec<SharedString>) {
+        self.theme_registry.remove_icon_themes(&icon_themes);
+    }
+
+    fn load_icon_theme(
+        &self,
+        icon_theme_path: PathBuf,
+        icons_root_dir: PathBuf,
+        fs: Arc<dyn Fs>,
+    ) -> Task<Result<()>> {
+        let theme_registry = self.theme_registry.clone();
+        self.executor.spawn(async move {
+            theme_registry
+                .load_icon_theme(&icon_theme_path, &icons_root_dir, fs)
+                .await
+        })
+    }
 }