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/gpui/src/gpui.rs

@@ -197,6 +197,11 @@ pub trait AppContext {
     where
         T: 'static;
 
+    /// Update a entity in the app context.
+    fn as_mut<'a, T>(&'a mut self, handle: &Entity<T>) -> Self::Result<GpuiBorrow<'a, T>>
+    where
+        T: 'static;
+
     /// Read a entity from the app context.
     fn read_entity<T, R>(
         &self,