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/editor/src/test.rs

@@ -108,7 +108,7 @@ pub fn assert_text_with_selections(
     assert_eq!(editor.text(cx), unmarked_text, "text doesn't match");
     let actual = generate_marked_text(
         &editor.text(cx),
-        &editor.selections.ranges(cx),
+        &editor.selections.ranges(&editor.display_snapshot(cx)),
         marked_text.contains("«"),
     );
     assert_eq!(actual, marked_text, "Selections don't match");