Correctly handle var, let, and const

[mwaldrich]

Currently, all 3 ways of declaring a variable are handled like var inside jankscripten. This stemmed from a parser bug in the old parser, and I had to replicate the bug in the new parser in order to get the tests to pass.

I'm not sure if we already decided to support let or const, but I don't think there's any machinery implemented for them already. We might be able to get away with treating const like var. let might be trickier to ignore, since it has different scoping rules than var, which some of our programs may rely on.