Support astral \u{...} outside of character classes without native /u

[slevithan]

XRegExp 3.0.0 introduced support for ES6's \u{...} syntax in all browsers, but requires native flag u if this syntax is used for astral code points--i.e. above \u{FFFF}. However, it would not require much work to support astral \u{...} outside of character classes using surrogate pairs. You'd simply need to generate the surrogate pair and wrap it with (?: ). The wrapping would allow it to work as expected with following quantifiers.

Unfortunately, an error would still need to be thrown if astral \u{...} is used within character classes and the regex does not use native u.

Con: Syntax that works only outside of character classes has potential to cause confusion or frustration.