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_preview/src/markdown_preview_view.rs

@@ -383,7 +383,7 @@ impl MarkdownPreviewView {
                 (contents, file_location)
             })?;
 
-            let parsing_task = cx.background_executor().spawn(async move {
+            let parsing_task = cx.background_spawn(async move {
                 parse_markdown(&contents, file_location, Some(language_registry)).await
             });
             let contents = parsing_task.await;