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/repl/src/outputs/markdown.rs

@@ -19,9 +19,8 @@ impl MarkdownView {
     pub fn from(text: String, cx: &mut Context<Self>) -> Self {
         let task = cx.spawn(|markdown_view, mut cx| {
             let text = text.clone();
-            let parsed = cx
-                .background_executor()
-                .spawn(async move { parse_markdown(&text, None, None).await });
+            let parsed =
+                cx.background_spawn(async move { parse_markdown(&text, None, None).await });
 
             async move {
                 let content = parsed.await;