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/file_finder/src/file_finder.rs

@@ -127,7 +127,7 @@ impl FileFinder {
                 let abs_path = abs_path?;
                 if project.is_local() {
                     let fs = fs.clone();
-                    Some(cx.background_executor().spawn(async move {
+                    Some(cx.background_spawn(async move {
                         if fs.is_file(&abs_path).await {
                             Some(FoundPath::new(project_path, Some(abs_path)))
                         } else {