Tokens in lookaround should not be added to character classes when interpolating a RegExp

[slevithan]

Example: RegExp.make`[${/(?=a)b/i}]` returns /[ABab]/, even though the a is only part of an assertion and not part of what the regex is capable of matching.

You could make a similar case for RegExp.make`[${/a{0}b/}]`, which returns /[ab]/, but maybe shouldn't include the a since it can't be matched. Same goes for RegExp.make`[${/|b/}]` where the b can never be matched. These cases might be unreasonable, but I think that all of these examples help illustrate the strangeness / non-intuitiveness of the design decision to make RegExp instances interpolated in character classes work this way (even though it's a cool feature).