For named groups that are not given, an Error will be emitted anyway.
This also handles the case when an Emitter for group `0` is provided
or not. Since numbers can also be used for names.
But it might be over-doing, because why would anyone use ByGroupNames
if they wanted to assign a token to the whole match?!
The engine was always passing a string sliced to the current position,
resulting in ^ always matching. Switched to use
FindRunesMatchStartingAt.
Fixes#242.
