feat: add wo_utf8 - UTF-8 string processing utilities
- wo_u8strlen: calculate character count in UTF-8 string
- wo_u8stridxstr: get pointer at character index
- wo_u8stridx: get wide character at index
- wo_u8substr: extract substring with byte length output
Adapted from woolang master branch (BiDuang/woolang) wo_utf8 module Converted from C++ to pure C for woort compatibility