Allow actions to be deserialized from JSON

Introduce separate macro for implementing 'internal' actions which
are not intended to be loaded from keymaps.

crates/gpui/src/gpui.rs

@@ -33,3 +33,6 @@ pub use platform::{Event, NavigationDirection, PathPromptOptions, Platform, Prom
 pub use presenter::{
     Axis, DebugContext, EventContext, LayoutContext, PaintContext, SizeConstraint, Vector2FExt,
 };
+
+pub use anyhow;
+pub use serde_json;