JS Regex Translation

Here's the machinery that translates DXR-flavored (Python- and PCRE-ish except without a few fancy feature like lookarounds (so far)) to JS ones we can run inside ES. Really, the only difference is that \c means something in JS's regexes but not in Python's. You can see in the tests that the ones expecting a difference are very few.
It's simply that they're useless for extracting static substrings from. That's all the SubstringTree is concerned with.