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/gpui/src/style.rs

@@ -3,8 +3,8 @@ use std::{iter, mem, ops::Range};
 use crate::{
     black, phi, point, quad, rems, AbsoluteLength, Bounds, ContentMask, Corners, CornersRefinement,
     CursorStyle, DefiniteLength, Edges, EdgesRefinement, ElementContext, Font, FontFeatures,
-    FontStyle, FontWeight, Hsla, Length, Pixels, Point, PointRefinement, Rgba, SharedString, Size,
-    SizeRefinement, Styled, TextRun,
+    FontStyle, FontWeight, Global, Hsla, Length, Pixels, Point, PointRefinement, Rgba,
+    SharedString, Size, SizeRefinement, Styled, TextRun,
 };
 use collections::HashSet;
 use refineable::Refineable;
@@ -20,6 +20,8 @@ pub use taffy::style::{
 /// GPUI.
 pub struct DebugBelow;
 
+impl Global for DebugBelow {}
+
 /// The CSS styling that can be applied to an element via the `Styled` trait
 #[derive(Clone, Refineable, Debug)]
 #[refineable(Debug)]