Parser improvements -- markdown inside json value

[aaronvg]

This one is hard to do, but wish we could parse all of finalAnswer correctly here:

Instead, our parser stops on the first closing curly found in the markdown.

the remedy is to tell people to use triple-quote or triple-backtic format, which is more resilient. But the parser should ideally know that it's trying to parse finalAnswer, which is a string, and start keeping track of opened/closed parentheses.

There's still corner cases where it would fail (if the parentheses or curlies don't match up perhaps), but more resilient.