feat(node): Use worker thread for ANR detection

[timfish]

I was looking to add more functionality to the ANR debugger but I've been having niggling feelings surrounding our use of a child process to do this. After much experimenting and debugging I managed to get it working via a worker thread and the inspector APIs connectToMainThread().

Also closes #9324

Pros:

• No extra processes
• Only ~20MB memory overhead (vs at least 60MB for a child process)
• Uses inspector API so we can remove the websockets implementation

Cons:

• Increases the minimum supported node version to v12.11.0
• This won't work in Electron since an Electron worker is a plain node thread and does not support Electron APIs.
  • We cannot load the app entry point into the worker like we do here in Node since it'll be using require('electron')
  • This isn't the end of the world since the Electron implementation needs some additional work (After enabling ANR detection for the main process, all Electron processes are spawned twice sentry-electron#784).

[timfish]

Now it's working via a worker thread, I'm tempted to resurrect my original load worker from base64 string idea 🤔

This has some pros:

• Will work in Electron since the worker is just pre-bundled Node.js code
• ANR can become just be an integration
• Lower memory usage since the worker doesn't have all the app imports
• Less chance of running the app twice

And some cons:

• Minimum supported Node version becomes v14.7
• Additional bundle size increase of worker code
• We'll need to pass all the context through to the worker since it will no longer be running the full sdk

[timfish]

Closing in favour of #9823