Reduce display_map snapshot creation (#39354)

Re-applies https://github.com/zed-industries/zed/pull/30840

This PR re-applies the initial
[PR](https://github.com/zed-industries/zed/pull/30840). As it was closed
because it was hard to land, because of the many conflicts. This PR
re-applies the changes for it.

In several cases we were creating multiple display_map
snapshots within the same root-level function call.
Creating a display_map snapshot is quite slow, and in some
cases we were creating the snapshot multiple times.

Release Notes:

- N/A

crates/debugger_ui/src/tests/new_process_modal.rs

@@ -231,7 +231,10 @@ async fn test_save_debug_scenario_to_file(executor: BackgroundExecutor, cx: &mut
 
     editor.update(cx, |editor, cx| {
         assert_eq!(
-            editor.selections.newest::<Point>(cx).head(),
+            editor
+                .selections
+                .newest::<Point>(&editor.display_snapshot(cx))
+                .head(),
             Point::new(5, 2)
         )
     });