cx.background_executor().spawn(...) -> cx.background_spawn(...) (#25103)

Done automatically with

> ast-grep -p '$A.background_executor().spawn($B)' -r
'$A.background_spawn($B)' --update-all --globs "\!crates/gpui"

Followed by:

* `cargo fmt`
* Unexpected need to remove some trailing whitespace.
* Manually adding imports of `gpui::{AppContext as _}` which provides
`background_spawn`
* Added `AppContext as _` to existing use of `AppContext`

Release Notes:

- N/A

crates/editor/src/element.rs

@@ -5264,8 +5264,7 @@ impl EditorElement {
                 Some(cx.spawn_in(window, |editor, mut cx| async move {
                     let scrollbar_size = scrollbar_layout.hitbox.size;
                     let scrollbar_markers = cx
-                        .background_executor()
-                        .spawn(async move {
+                        .background_spawn(async move {
                             let max_point = snapshot.display_snapshot.buffer_snapshot.max_point();
                             let mut marker_quads = Vec::new();
                             if scrollbar_settings.git_diff {