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/image_viewer/src/image_viewer.rs

@@ -252,7 +252,7 @@ impl SerializableItem for ImageView {
         let workspace_id = workspace.database_id()?;
         let image_path = self.image_item.read(cx).file.as_local()?.abs_path(cx);
 
-        Some(cx.background_executor().spawn({
+        Some(cx.background_spawn({
             async move {
                 IMAGE_VIEWER
                     .save_image_path(item_id, workspace_id, image_path)