Feature: HalfBracket

Make it so when you type an open paren, you get just an open paren, and only when you then type a close paren, do you get a paren group that will resize to match the enclosed math. In the same vein, backspacing over a paren group deletes just the one paren, the matched opposite goes back to being just a lone open paren.

I like that this works really well backwards (which is really common, right? I so often type something and then realize I want the last 2 terms in parens), it's nice to be able to type the close paren first and then the open paren; I like that this extends trivially to [mis-matched bracket groups](https://github.com/mathquill/mathquill/issues/150) like [a, b); but best of all, I feel like, especially in terms of a "low floor" for someone who's never typed in MathQuill before, this is as close as you can get to feeling like you're typing normal text that just magically does what you want.

Like basically, you type parens like in normal text and whenever they happen to be a matched pair, they will magically resize for the enclosed math.