Add delete tracking and symlink/unlink to OverlayFS (#8857)

* Add symlink to FileSystem interface

* Add CopyOnWriteToMemoryFS test util

* Allow deep symlinks

* Add tests; fix bugs

* Fix tracking deletes/unlinks for symlinks

* Move CopyOnWriteToMemoryfS to @parcel/fs

Also make it serialiable for multithread

* lint

* Clean up some nits and oversights

* Merge CopyOnWriteToMemoryFS into OverlayFS

* Fix tests

* Fix tests

* Fix copy on write for nested paths

* Fix chdir behavior

Author: lettertwo

packages/core/fs/src/NodeFS.js

@@ -34,6 +34,7 @@ export class NodeFS implements FileSystem {
   copyFile: any = promisify(fs.copyFile);
   stat: any = promisify(fs.stat);
   readdir: any = promisify(fs.readdir);
+  symlink: any = promisify(fs.symlink);
   unlink: any = promisify(fs.unlink);
   utimes: any = promisify(fs.utimes);
   ncp: any = promisify(ncp);