This fixes #21434 for `BroadcastChannel` and `WebSocketStream`.
`--unstable` still enable both, but granular unstable flags now also
work:
* `--unstable-net` now enables `WebSocketStream`.
* `--unstable-broadcast-channel` now enables `BroadcastChannel`.
* Additionally, there are now tests for all granular unstable flags.
Since `unsafe-proto` already had tests, so I didn't add any for this
one.
It also introduces a map to keep track of granular unstable ids without
having to sync multiple places.
