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/repl/src/notebook/cell.rs

@@ -134,8 +134,7 @@ impl Cell {
 
                         cx.spawn_in(window, |this, mut cx| async move {
                             let parsed_markdown = cx
-                                .background_executor()
-                                .spawn(async move {
+                                .background_spawn(async move {
                                     parse_markdown(&source, None, Some(languages)).await
                                 })
                                 .await;