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>

crates/gpui/src/platform/mac/attributed_string.rs

@@ -70,9 +70,7 @@ mod tests {
 
         unsafe {
             let image: id = msg_send![class!(NSImage), alloc];
-            image.initWithContentsOfFile_(
-                NSString::alloc(nil).init_str("/Users/rtfeldman/Downloads/test.jpeg"),
-            );
+            image.initWithContentsOfFile_(NSString::alloc(nil).init_str("test.jpeg"));
             let _size = image.size();
 
             let string = NSString::alloc(nil).init_str("Test String");