compute: add resolver to construct TextResult

Created by: rvantonder

As per title.

• MatchContext is the one kind of result we support on the compute endpoint (variable environments).
• TextResult is the other, but we don't construct this kind of value yet. This function constructs it when called by something that generates such a result (upcoming replace command).

Hopefully the linter doesn't whine that this is dead. I'll convince it if it does.