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/extension/src/extension_host_proxy.rs

@@ -103,6 +103,21 @@ pub trait ExtensionThemeProxy: Send + Sync + 'static {
     fn load_user_theme(&self, theme_path: PathBuf, fs: Arc<dyn Fs>) -> Task<Result<()>>;
 
     fn reload_current_theme(&self, cx: &mut AppContext);
+
+    fn list_icon_theme_names(
+        &self,
+        icon_theme_path: PathBuf,
+        fs: Arc<dyn Fs>,
+    ) -> Task<Result<Vec<String>>>;
+
+    fn remove_icon_themes(&self, icon_themes: Vec<SharedString>);
+
+    fn load_icon_theme(
+        &self,
+        icon_theme_path: PathBuf,
+        icons_root_dir: PathBuf,
+        fs: Arc<dyn Fs>,
+    ) -> Task<Result<()>>;
 }
 
 impl ExtensionThemeProxy for ExtensionHostProxy {
@@ -137,6 +152,39 @@ impl ExtensionThemeProxy for ExtensionHostProxy {
 
         proxy.reload_current_theme(cx)
     }
+
+    fn list_icon_theme_names(
+        &self,
+        icon_theme_path: PathBuf,
+        fs: Arc<dyn Fs>,
+    ) -> Task<Result<Vec<String>>> {
+        let Some(proxy) = self.theme_proxy.read().clone() else {
+            return Task::ready(Ok(Vec::new()));
+        };
+
+        proxy.list_icon_theme_names(icon_theme_path, fs)
+    }
+
+    fn remove_icon_themes(&self, icon_themes: Vec<SharedString>) {
+        let Some(proxy) = self.theme_proxy.read().clone() else {
+            return;
+        };
+
+        proxy.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 Some(proxy) = self.theme_proxy.read().clone() else {
+            return Task::ready(Ok(()));
+        };
+
+        proxy.load_icon_theme(icon_theme_path, icons_root_dir, fs)
+    }
 }
 
 pub trait ExtensionGrammarProxy: Send + Sync + 'static {