fix: remove awareness stats on page unload instead of beforeunload

[iowxy]

I notice the code in file packages/provider/src/HocuspocusProvider.ts

window.addEventListener('beforeunload', this.boundBeforeUnload)

I must say that what if I prevent the page to be close? consider this logic below

// https://developer.mozilla.org/en/docs/Web/API/Window/beforeunload_event
window.addEventListener('beforeunload', (event) => {
  event.preventDefault()
  event.returnValue = ''
})

Develop can write intercept logic and users may cancel to close this page.

[janthurau]

Totally makes sense. Thanks for this!

I guess we added beforeUnload to make sure the awareness message can actually arrive at the server, but seems this works fine (yjs/y-websocket#119).