Available since: Gideros 2016.06
Returns the position (in bytes) where the encoding of the n-th character of s (counting from position i) starts.
(number) = utf8.offset(s,n,i)
A negative n gets characters before position i.
The default for i is 1 when n is non-negative and #s + 1 otherwise, so that utf8.offset(s, -n) gets the offset of the n-th character from the end of the string.
If the specified character is neither in the subject nor right after its end, the function returns nil.
As a special case, when n is 0 the function returns the start of the encoding of the character that contains the i-th byte of s.
This function assumes that s is a valid UTF-8 string.
i: (number) optional
Returns (number) position (in bytes)