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/semantic_index/examples/index.rs

@@ -1,6 +1,6 @@
 use client::Client;
 use futures::channel::oneshot;
-use gpui::{App, Global};
+use gpui::App;
 use language::language_settings::AllLanguageSettings;
 use project::Project;
 use semantic_index::{OpenAiEmbeddingModel, OpenAiEmbeddingProvider, SemanticIndex};