macOS: make Ime::Preedit cursor range surrogate-safe in setMarkedText

`setMarkedText:selectedRange:replacementRange:` converted the IME's UTF-16
`selectedRange` into UTF-8 byte offsets by taking `substringToIndex:` prefixes
and measuring them with `NSString::len()`
(`lengthOfBytesUsingEncoding:NSUTF8StringEncoding`). When an index falls inside
a surrogate pair, the prefix ends in a lone high surrogate, which UTF-8 cannot
represent, so `lengthOfBytesUsingEncoding:` returns 0 for the entire prefix and
the offset silently collapses to 0.