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/project_panel/src/file_associations.rs

@@ -2,7 +2,7 @@ use std::{path::Path, str, sync::Arc};
 
 use collections::HashMap;
 
-use gpui::{AppContext, AssetSource};
+use gpui::{AppContext, AssetSource, Global};
 use serde_derive::Deserialize;
 use util::{maybe, paths::PathExt};
 
@@ -17,6 +17,8 @@ pub struct FileAssociations {
     types: HashMap<String, TypeConfig>,
 }
 
+impl Global for FileAssociations {}
+
 const COLLAPSED_DIRECTORY_TYPE: &'static str = "collapsed_folder";
 const EXPANDED_DIRECTORY_TYPE: &'static str = "expanded_folder";
 const COLLAPSED_CHEVRON_TYPE: &'static str = "collapsed_chevron";