Strict mode detection logic

[adams85]

Hi there,

First of all, hats off to you as this a very solid, well-written project you're working on here. I've ported it to .NET recently and while doing that, I went through most of the codebase, so I'm saying this from experience.

Apart from some minor issues (will open additional issues/PRs regarding those), I only found one part that didn't really click with me: the strict mode detection logic. It's not something very problematic, just feels a bit fragile. And you can actually break it if you really want to. E.g. the following construct tricks Acorn into detecting strict mode incorrectly:

function f() { 'use strict'
in a; 00 }

So, I replaced this part with a more robust detection logic, and I'd recommend you the same.