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/theme/src/settings.rs

@@ -2,7 +2,8 @@ use crate::one_themes::one_dark;
 use crate::{SyntaxTheme, Theme, ThemeRegistry, ThemeStyleContent};
 use anyhow::Result;
 use gpui::{
-    px, AppContext, Font, FontFeatures, FontStyle, FontWeight, Pixels, Subscription, ViewContext,
+    px, AppContext, Font, FontFeatures, FontStyle, FontWeight, Global, Pixels, Subscription,
+    ViewContext,
 };
 use refineable::Refineable;
 use schemars::{
@@ -34,6 +35,8 @@ pub struct ThemeSettings {
 #[derive(Default)]
 pub(crate) struct AdjustedBufferFontSize(Pixels);
 
+impl Global for AdjustedBufferFontSize {}
+
 #[derive(Clone, Debug, Default, Serialize, Deserialize, JsonSchema)]
 pub struct ThemeSettingsContent {
     #[serde(default)]