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/assistant2/src/assistant2.rs

@@ -11,7 +11,7 @@ use feature_flags::FeatureFlagAppExt as _;
 use futures::{future::join_all, StreamExt};
 use gpui::{
     list, prelude::*, AnyElement, AppContext, AsyncWindowContext, EventEmitter, FocusHandle,
-    FocusableView, Global, ListAlignment, ListState, Render, Task, View, WeakView,
+    FocusableView, ListAlignment, ListState, Render, Task, View, WeakView,
 };
 use language::{language_settings::SoftWrap, LanguageRegistry};
 use open_ai::{FunctionContent, ToolCall, ToolCallContent};

crates/assistant2/src/completion_provider.rs

@@ -2,7 +2,7 @@ use anyhow::Result;
 use assistant_tooling::ToolFunctionDefinition;
 use client::{proto, Client};
 use futures::{future::BoxFuture, stream::BoxStream, FutureExt, StreamExt};
-use gpui::Global;
+use gpui::{AppContext, Global};
 use std::sync::Arc;
 
 pub use open_ai::RequestMessage as CompletionMessage;
@@ -11,6 +11,10 @@ pub use open_ai::RequestMessage as CompletionMessage;
 pub struct CompletionProvider(Arc<dyn CompletionProviderBackend>);
 
 impl CompletionProvider {
+    pub fn get(cx: &AppContext) -> &Self {
+        cx.global::<CompletionProvider>()
+    }
+
     pub fn new(backend: impl CompletionProviderBackend) -> Self {
         Self(Arc::new(backend))
     }