gpui: Add Global marker trait (#7095)

This should prevent a class of bugs where one queries the wrong type of
global, which results in oddities at runtime.

Release Notes:

- N/A

---------

Co-authored-by: Marshall <marshall@zed.dev>
Co-authored-by: Marshall Bowers <elliott.codes@gmail.com>

crates/feature_flags/src/feature_flags.rs

@@ -1,4 +1,4 @@
-use gpui::{AppContext, Subscription, ViewContext};
+use gpui::{AppContext, Global, Subscription, ViewContext};
 
 #[derive(Default)]
 struct FeatureFlags {
@@ -12,6 +12,8 @@ impl FeatureFlags {
     }
 }
 
+impl Global for FeatureFlags {}
+
 pub trait FeatureFlag {
     const NAME: &'static str;
 }