gpui: Add use state APIs (#34741)

This PR adds a component level state API to GPUI, as well as a few
utilities for simplified interactions with entities

Release Notes:

- N/A

crates/eval/src/example.rs

@@ -422,6 +422,13 @@ impl AppContext for ExampleContext {
         self.app.update_entity(handle, update)
     }
 
+    fn as_mut<'a, T>(&'a mut self, handle: &Entity<T>) -> Self::Result<gpui::GpuiBorrow<'a, T>>
+    where
+        T: 'static,
+    {
+        self.app.as_mut(handle)
+    }
+
     fn read_entity<T, R>(
         &self,
         handle: &Entity<T>,