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_builder.rs

@@ -560,6 +560,21 @@ fn populate_defaults(manifest: &mut ExtensionManifest, extension_path: &Path) ->
         }
     }
 
+    let icon_themes_dir = extension_path.join("icon_themes");
+    if icon_themes_dir.exists() {
+        for entry in fs::read_dir(&icon_themes_dir).context("failed to list icon themes dir")? {
+            let entry = entry?;
+            let icon_theme_path = entry.path();
+            if icon_theme_path.extension() == Some("json".as_ref()) {
+                let relative_icon_theme_path =
+                    icon_theme_path.strip_prefix(extension_path)?.to_path_buf();
+                if !manifest.icon_themes.contains(&relative_icon_theme_path) {
+                    manifest.icon_themes.push(relative_icon_theme_path);
+                }
+            }
+        }
+    }
+
     let snippets_json_path = extension_path.join("snippets.json");
     if snippets_json_path.exists() {
         manifest.snippets = Some(snippets_json_path);

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 {

crates/extension/src/extension_manifest.rs

@@ -70,6 +70,8 @@ pub struct ExtensionManifest {
     #[serde(default)]
     pub themes: Vec<PathBuf>,
     #[serde(default)]
+    pub icon_themes: Vec<PathBuf>,
+    #[serde(default)]
     pub languages: Vec<PathBuf>,
     #[serde(default)]
     pub grammars: BTreeMap<Arc<str>, GrammarManifestEntry>,
@@ -199,6 +201,7 @@ fn manifest_from_old_manifest(
             themes.dedup();
             themes
         },
+        icon_themes: Vec::new(),
         languages: {
             let mut languages = manifest_json.languages.into_values().collect::<Vec<_>>();
             languages.sort();