Closes #40359

We were segfaulting when opening a UTF-8 file starting with a byte order
mark due to a mismatch in our UTF-16 indexing calculations caused by
Core Foundations `replace_str` stripping the BOM internally. This PR
fixes the crash by replacing one of our manual calculations by calling
the Core Foundations API to get the length of a string.

Release Notes:

- Fixed a crash on macOS when opening a file that starts with a UTF-8
byte order mark (BOM).

Co-authored-by: HactarCE <6060305+HactarCE@users.noreply.github.com>