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/editor/src/scroll.rs

@@ -10,7 +10,7 @@ use crate::{
     MultiBufferSnapshot, ToPoint,
 };
 pub use autoscroll::{Autoscroll, AutoscrollStrategy};
-use gpui::{point, px, AppContext, Entity, Pixels, Task, ViewContext};
+use gpui::{point, px, AppContext, Entity, Global, Pixels, Task, ViewContext};
 use language::{Bias, Point};
 pub use scroll_amount::ScrollAmount;
 use std::{
@@ -27,6 +27,8 @@ const SCROLLBAR_SHOW_INTERVAL: Duration = Duration::from_secs(1);
 #[derive(Default)]
 pub struct ScrollbarAutoHide(pub bool);
 
+impl Global for ScrollbarAutoHide {}
+
 #[derive(Clone, Copy, Debug, PartialEq)]
 pub struct ScrollAnchor {
     pub offset: gpui::Point<f32>,