extensions_ui: Add ability to open the extensions view with a pre-selected filter (#27093)

This PR adds the ability to open the extensions view via the `zed:
extensions` action with a pre-selected filter.

The "Install Themes" and "Install Icon Themes" buttons in their
respective selectors take advantage of this to set the filter when
opening the view:


https://github.com/user-attachments/assets/2e345c0f-418a-47b6-811e-cabae6c616d1

Release Notes:

- N/A

crates/zed_actions/src/lib.rs

@@ -35,12 +35,32 @@ actions!(
         Quit,
         OpenKeymap,
         About,
-        Extensions,
         OpenLicenses,
         OpenTelemetryLog,
     ]
 );
 
+#[derive(PartialEq, Clone, Copy, Debug, Deserialize, JsonSchema)]
+#[serde(rename_all = "snake_case")]
+pub enum ExtensionCategoryFilter {
+    Themes,
+    IconThemes,
+    Languages,
+    Grammars,
+    LanguageServers,
+    ContextServers,
+    SlashCommands,
+    IndexedDocsProviders,
+    Snippets,
+}
+
+#[derive(PartialEq, Clone, Default, Debug, Deserialize, JsonSchema)]
+pub struct Extensions {
+    /// Filters the extensions page down to extensions that are in the specified category.
+    #[serde(default)]
+    pub category_filter: Option<ExtensionCategoryFilter>,
+}
+
 #[derive(PartialEq, Clone, Default, Debug, Deserialize, JsonSchema)]
 pub struct DecreaseBufferFontSize {
     #[serde(default)]
@@ -80,6 +100,7 @@ pub struct ResetUiFontSize {
 impl_actions!(
     zed,
     [
+        Extensions,
         DecreaseBufferFontSize,
         IncreaseBufferFontSize,
         ResetBufferFontSize,