(Reminder) Switch to BroadcastChannel

As of commit https://github.com/tmpmachine/codetmp/commit/40490c9fe91343ff7a100f58ac7cb7c22630b5e1, switch to simple implementation of BroadcastChannel to allow cross origin messaging due to modern browser's Storage Partitioning update.

Currently have little browser support: https://developer.mozilla.org/en-US/docs/Web/API/Document/requestStorageAccess#browser_compatibility

In the future, support both BroadcastChannel and MessageChannel.