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/markdown/src/markdown.rs

@@ -178,7 +178,7 @@ impl Markdown {
 
         let text = self.source.clone();
         let parse_text_only = self.options.parse_links_only;
-        let parsed = cx.background_executor().spawn(async move {
+        let parsed = cx.background_spawn(async move {
             let text = SharedString::from(text);
             let events = match parse_text_only {
                 true => Arc::from(parse_links_only(text.as_ref())),