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/symbols_command.rs

@@ -4,7 +4,7 @@ use assistant_slash_command::{
     SlashCommandResult,
 };
 use editor::Editor;
-use gpui::{Task, WeakEntity};
+use gpui::{AppContext as _, Task, WeakEntity};
 use language::{BufferSnapshot, LspAdapterDelegate};
 use std::sync::Arc;
 use std::{path::Path, sync::atomic::AtomicBool};
@@ -69,7 +69,7 @@ impl SlashCommand for OutlineSlashCommand {
             let snapshot = buffer.read(cx).snapshot();
             let path = snapshot.resolve_file_path(cx, true);
 
-            cx.background_executor().spawn(async move {
+            cx.background_spawn(async move {
                 let outline = snapshot
                     .outline(None)
                     .context("no symbols for active tab")?;