gpui: Improve Global ergonomics (#11923)

This PR adds some ergonomic improvements when working with GPUI
`Global`s.

Two new traits have been added—`ReadGlobal` and `UpdateGlobal`—that
provide associated functions on any type that implements `Global` for
accessing and updating the global without needing to call the methods on
the `cx` directly (which generally involves qualifying the type).

I looked into adding `ObserveGlobal` as well, but this seems a bit
trickier to implement as the signatures of `cx.observe_global` vary
slightly between the different contexts.

Release Notes:

- N/A

crates/gpui/src/gpui.rs

@@ -77,6 +77,7 @@ mod element;
 mod elements;
 mod executor;
 mod geometry;
+mod global;
 mod input;
 mod interactive;
 mod key_dispatch;
@@ -125,6 +126,7 @@ pub use element::*;
 pub use elements::*;
 pub use executor::*;
 pub use geometry::*;
+pub use global::*;
 pub use gpui_macros::{register_action, test, IntoElement, Render};
 pub use input::*;
 pub use interactive::*;
@@ -327,15 +329,3 @@ impl<T> Flatten<T> for Result<T> {
         self
     }
 }
-
-/// A marker trait for types that can be stored in GPUI's global state.
-///
-/// This trait exists to provide type-safe access to globals by restricting
-/// the scope from which they can be accessed. For instance, the actual type
-/// that implements [`Global`] can be private, with public accessor functions
-/// that enforce correct usage.
-///
-/// Implement this on types you want to store in the context as a global.
-pub trait Global: 'static {
-    // This trait is intentionally left empty, by virtue of being a marker trait.
-}