fs: Replace a bunch of uses of smol::fs with manual impls (#40172)

smol::fs uses a separate threadpool, which is a bit yuck.

This PR also added a benchmark you can use to run a full worktree scan
(initial one, that is) for arbitrary worktree.. and refactored worktree
scanner to use async locks, as otherwise tests were deadlocking. :)
I've benchmarked it against Zed, Linux and Chromium and saw a ~60% drop
in initial worktree scan times across the board.
Release Notes:

- Significantly (3.3x speedup over the old implementation) improved
speed of Zed's worktree scanner, that's responsible for synchronizing
the state of your project with the state of files on hard drive.

---------

Co-authored-by: Smit Barmase <heysmitbarmase@gmail.com>

crates/editor/src/editor_tests.rs

@@ -12509,6 +12509,7 @@ async fn test_strip_whitespace_and_format_via_lsp(cx: &mut TestAppContext) {
     )
     .await;
 
+    cx.run_until_parked();
     // Set up a buffer white some trailing whitespace and no trailing newline.
     cx.set_state(
         &[