editor: Put more syntax walking features on the background (#48450)

Currently we always compute breadcrumbs and sticky headers on every
editor paint which is not cheap to do especially in bigger files, moving
this off to be computed on event handling where they change and then
caching them can save serveral milliseconds per render in bigger files.

This also puts matching brackets refreshing and document highlights on a
background task, as this tends to block the main task for prolonged time
as well.

Release Notes:

- N/A *or* Added/Fixed/Improved ...

crates/git_ui/src/multi_diff_view.rs

@@ -13,7 +13,6 @@ use std::{
     path::{Path, PathBuf},
     sync::Arc,
 };
-use theme;
 use ui::{Color, Icon, IconName, Label, LabelCommon as _};
 use util::paths::PathStyle;
 use util::rel_path::RelPath;
@@ -339,8 +338,8 @@ impl Item for MultiDiffView {
         ToolbarItemLocation::PrimaryLeft
     }
 
-    fn breadcrumbs(&self, theme: &theme::Theme, cx: &App) -> Option<Vec<BreadcrumbText>> {
-        self.editor.breadcrumbs(theme, cx)
+    fn breadcrumbs(&self, cx: &App) -> Option<Vec<BreadcrumbText>> {
+        self.editor.breadcrumbs(cx)
     }
 
     fn added_to_workspace(