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/assistant_slash_commands/src/file_command.rs

@@ -156,7 +156,7 @@ impl SlashCommand for FileSlashCommand {
             cx,
         );
         let comment_id = cx.theme().syntax().highlight_id("comment").map(HighlightId);
-        cx.background_executor().spawn(async move {
+        cx.background_spawn(async move {
             Ok(paths
                 .await
                 .into_iter()