Add a toRegex helper method on Query.TermRegex

[valencik]

If an application is going to use Query.TermRegex it's likely they'll want to turn the captured string into a Regex object.
Doing so can throw a PatternSyntaxException exception.
We should provide a method that attempts to create the Regex in an Either and turns any PatternSyntaxException exception into a Left:

def toRegex: Either[String, Regex] = ???