title_bar: Remove dependency on extensions_ui (#20929)

This PR removes a dependency on the `extensions_ui` from the `title_bar`
crate.

This dependency only existed to reference the `Extensions` action, which
has now been moved to the `zed_actions` crate.

This allows `title_bar` to move up in the crate dependency graph.

Release Notes:

- N/A

crates/zed_actions/src/lib.rs

@@ -32,6 +32,7 @@ actions!(
         Quit,
         OpenKeymap,
         About,
+        Extensions,
         OpenLicenses,
         OpenTelemetryLog,
         DecreaseBufferFontSize,