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/workspace/Cargo.toml

@@ -43,6 +43,7 @@ async-recursion = "1.0.0"
 itertools = "0.10"
 bincode = "1.2.1"
 anyhow.workspace = true
+derive_more.workspace = true
 futures.workspace = true
 lazy_static.workspace = true
 log.workspace = true