Update protobufjs from v5 to v7 (#21)

API changes required for v7:
- loadProto().build() -> parse().root.lookupType()/lookupEnum().values
- new Type() -> Type.create()
- .toArrayBuffer() -> .encode().finish()
- .decode(buf) -> .decode(new Uint8Array(buf))
- snake_case -> camelCase field names (automatic in v7)
- ByteBuffer -> Uint8Array for bytes fields

Behavior change for empty bytes handling:
- Check hello.challenge && hello.challenge.length because v7 returns
  empty Uint8Array (truthy) for unset bytes fields instead of null

Author: stefanrammo