Bug 250839

I've created a git repo with a test project demonstrating the bug here: https://github.com/shareup/sharedworker-restart-webkit-bug

To replace or restart a `SharedWorker`, the shared worker can call `self.close()` on itself, then a window can use `new SharedWorker('worker.js')` to create and connect to a new worker. 

The expectation is `new SharedWorker('worker.js')` will always either connect to an existing, running `SharedWorker`, or boot one. It's important to be able to replace a `SharedWorker` when non-backwards compatible code is deployed, and still have the guarantee that there is only ever zero or one shared worker is running (by always using the same pathname `'worker.js'`).

After `SharedWorkerGlobalScope.close()` is called, any other window which uses `new SharedWorker(worker.js)` will get a broken `SharedWorker`. I can see an HTTP request is made for `worker.js` (I don't have any headers which would cache the request), so WebKit is correctly loading the file from the network. However, it appears to never start up the shared worker or connect. 

The only remedy appears to be to close all tabs/windows which ever connected to the original `SharedWorker` (which is now closed), then `new SharedWorker('worker.js')` in a new window will work as expected again. 

I've tested in Safari 16 on macOS and iOS and in Safari Tech Preview on macOS and they all work the way I describe above.

The test in the linked repo will close, and recreate the same `SharedWorker` 5 times in the same window session and works correctly in other UAs which support `SharedWorker`, but not in WebKit.