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/default_command.rs

@@ -54,7 +54,7 @@ impl SlashCommand for DefaultSlashCommand {
         cx: &mut App,
     ) -> Task<SlashCommandResult> {
         let store = PromptStore::global(cx);
-        cx.background_executor().spawn(async move {
+        cx.background_spawn(async move {
             let store = store.await?;
             let prompts = store.default_prompt_metadata();