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/project/src/environment.rs

@@ -135,8 +135,7 @@ impl ProjectEnvironment {
 
         cx.spawn(|this, mut cx| async move {
             let (mut shell_env, error_message) = cx
-                .background_executor()
-                .spawn({
+                .background_spawn({
                     let worktree_abs_path = worktree_abs_path.clone();
                     async move {
                         load_worktree_shell_environment(&worktree_abs_path, &load_direnv).await