Fixes jashkenas#5428

This fix is simple enough but it causes a substantial change in behavior for Heregexes.

`#` inside of a character class shouldn't be considered a comment. By treating `#`
without whitespace in front as non-comments we could have slight compatability with
Python.

There are other places in the CoffeeScript source where people avoided escaping the `#`
by keeping it next to non-whitespace characters even outside of a character class. This
is different than how Python does it and is probably a bug but maybe it is too late with
the de facto CoffeeScript2 behavior.

I'm not sure if this should be merged in since it changes the behavior quite a lot. Maybe
another one for the CoffeeScript3/Civet bucket.

Refs
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Python Docs: https://docs.python.org/3/library/re.html#re.X