We can't keep `syntect::parsing::SyntaxReference` as part of the public
API, because that might prevent us from bumping to syntect 6.0.0 without
also bumping bat to v2.0.0, once we reach v1.0.0.
So introduce a new stripped down struct `Syntax` and return that
instead. Let it be fully owned to make the API simple. It is not going
to be in a hot code path anyway.
I have looked at all code of our 27 dependents but I can't find a single
instance of this method being used, so this change should be safe for
v1.0.0.
