Add SQL/JSON ON MISMATCH clause to JSON_VALUE

Hello hackers,

Here's a patch that attempts to $subject.

My original motivation was to implement this for JSON_TABLE,
but I realized it would be better to start with a smaller scope
by targeting the basic query functions first.

Currently this v1 passes the tests I've put for JSON_VALUE.

I've had trouble making JSON_QUERY work too,
so I suspect that although my tests pass,
there may be something I'm missing in the executor side of things.
While I troubleshoot that it'd be nice to get some feedback for this
version.

Regards,
Flo