Add support for escaped UTF-16 surrogate pairs

[steveh]

I have followed the convention above that error handling is TODO, but I can start implementing it if you like.

References:

• https://russellcottrell.com/greek/utilities/SurrogatePairCalculator.htm
• https://mathiasbynens.be/notes/javascript-unicode
• https://en.wikipedia.org/wiki/UTF-16#U+D800_to_U+DFFF_(surrogates)