Fix file descriptors leak in evals (#18351)

Fixes an issue where evals were hitting "too many open files" errors
because we were adding (and detaching) new directory watches for each
project. Now we add those watches globally/at the worktree level, and we
store the tasks so they stop watching on drop.

Release Notes:

- N/A

---------

Co-authored-by: Max <max@zed.dev>
Co-authored-by: Piotr <piotr@zed.dev>
Co-authored-by: Piotr Osiewicz <24362066+osiewicz@users.noreply.github.com>

Cargo.lock

@@ -10498,6 +10498,7 @@ dependencies = [
  "futures 0.3.30",
  "gpui",
  "parking_lot",
+ "paths",
  "serde",
  "serde_json",
  "snippet",