Clean up the `isJS` and `isWasm` booleans from the Runtime wire format

[jakemac53]

I am faking them for now when serializing, to maintain backwards compatibility with older flutter versions, when a user doesn't have a (transitive) dependency on test_api. But we should delete them after some period of time (TBD).

[jakemac53]

Blocked on https://github.com/flutter/flutter/issues/91107 specifically although we should maintain this backward compatibility for at least one stable version of flutter.