Turbopack: Use an async lock in the filesystem watcher (#92906)
We know that we have a lot of lock contention when creating watchers, because many watchers are often set up at roughly the same time near the start of building a route. Internal discussion thread: https://vercel.slack.com/archives/C09R44U5HQW/p1776271283799709
@lukesandberg's suggestion was that the lock is held over syscalls and file IO, so it's possible that this could starve the Tokio worker threads, so we should use an async lock so that we can yield back the worker thread.
Because this would simply unblock *other* work, and because this codepath is only used in `dev`, there's no sane way to benchmark or measure this, so we'll just have to hope and pray it's an improvement.
This PR is entirely AI-generated.
