Add JS_NewTwoByteString

[bnoordhuis]

Fixes: #992

[Gottox]

Just browsing the PRs and I must do what every internet user needs to do: Give unqualified opinions. But as this is a public API, it might make sense to comment.

I found the function name not clear. At first reading I thought it was a (somehow strange) optimization of Strings that are two bytes long. I know this under the term 'wide string', which also is reflected in the wchar_t datatype, so JS_NewWideString may make sense here to me.

[saghul]

I believe the name is aligned with how other JS engines name similar things.

[bnoordhuis]

Two-byte is what V8 calls it, deliberately, to make it clear it's not guaranteed to be UTF-16. "Wide" is not clearer, IMO, because how wide?

[chqrlie]

Two-byte is what V8 calls it, deliberately, to make it clear it's not guaranteed to be UTF-16. "Wide" is not clearer, IMO, because how wide?

JS_New16BitString() would be more explicit...