feat: add beforeunload event

[grantfitzsimmons]

see https://developer.mozilla.org/en-US/docs/Web/API/Window/beforeunload_event

Fixes #7504

Checklist

• Self-review the PR after opening it to make sure the changes look good and
  self-explanatory (or properly documented)
• Add relevant issue to release milestone
• Add pr to documentation list
• Add automated tests
• Add a reverse migration if a migration is present in the PR

Testing instructions