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/semantic_index/Cargo.toml

@@ -17,6 +17,7 @@ language = { path = "../language" }
 project = { path = "../project" }
 workspace = { path = "../workspace" }
 util = { path = "../util" }
+release_channel = { path = "../release_channel" }
 rpc = { path = "../rpc" }
 settings = { path = "../settings" }
 anyhow.workspace = true