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/cargo_workspace_command.rs

@@ -132,7 +132,7 @@ impl SlashCommand for CargoWorkspaceSlashCommand {
             let project = workspace.project().clone();
             let fs = workspace.project().read(cx).fs().clone();
             let path = Self::path_to_cargo_toml(project, cx);
-            let output = cx.background_executor().spawn(async move {
+            let output = cx.background_spawn(async move {
                 let path = path.with_context(|| "Cargo.toml not found")?;
                 Self::build_message(fs, &path).await
             });