From any UTF-16 offset, find the corresponding Str

问题:

My goal: given an arbitrary UTF-16 position in a String, find the corresponding String.Index that represents the Character (i.e. the extended grapheme cluster) the specified UTF-16 code unit is a part of.

Example:

(I put the code in a Gist for easy copying and pasting.)

This is my test string:

let str = "