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/util/src/util.rs

@@ -1,5 +1,4 @@
 pub mod arc_cow;
-pub mod channel;
 pub mod fs;
 pub mod github;
 pub mod http;