gpui: Fix Global trait (#11187)

This PR restores the `Global` trait's status as a marker trait.

This was the original intent from #7095, when it was added, that had
been lost in #9777.

The purpose of the `Global` trait is to statically convey what types can
and can't be accessed as `Global` state, as well as provide a way of
restricting access to said globals.

For example, in the case of the `ThemeRegistry` we have a private
`GlobalThemeRegistry` that is marked as `Global`:
91b3c24ed3/crates/theme/src/registry.rs (L25-L34)

We're then able to permit reading the `ThemeRegistry` from the
`GlobalThemeRegistry` via a custom getter, while still restricting which
callers are able to mutate the global:
91b3c24ed3/crates/theme/src/registry.rs (L46-L61)

Release Notes:

- N/A

crates/settings/src/settings_store.rs

@@ -207,6 +207,13 @@ trait AnySettingValue: 'static + Send + Sync {
 struct DeserializedSetting(Box<dyn Any>);
 
 impl SettingsStore {
+    pub fn update<C, R>(cx: &mut C, f: impl FnOnce(&mut Self, &mut C) -> R) -> R
+    where
+        C: BorrowAppContext,
+    {
+        cx.update_global(f)
+    }
+
     /// Add a new type of setting to the store.
     pub fn register_setting<T: Settings>(&mut self, cx: &mut AppContext) {
         let setting_type_id = TypeId::of::<T>();