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/file_icons/src/file_icons.rs

@@ -31,6 +31,10 @@ pub fn init(assets: impl AssetSource, cx: &mut AppContext) {
 }
 
 impl FileIcons {
+    pub fn get(cx: &AppContext) -> &Self {
+        cx.global::<FileIcons>()
+    }
+
     pub fn new(assets: impl AssetSource) -> Self {
         assets
             .load("icons/file_icons/file_types.json")