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/settings/Cargo.toml

@@ -17,6 +17,7 @@ collections = { path = "../collections" }
 gpui = { path = "../gpui" }
 fs = { path = "../fs" }
 feature_flags = { path = "../feature_flags" }
+release_channel = { path = "../release_channel" }
 util = { path = "../util" }
 
 anyhow.workspace = true