Using String#codePointAt()?

I noticed that the code converts utf8 to utf16 manually, however `String#codePointAt()` is already [standardized](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/codePointAt) for quite a while.

I think switch to use the built in version should give unidecode a huge performance boost.
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