Use str::get instead of a byte-length check plus slice when parsing the
uniXXXX glyph-name form. The byte-length guard only proved the index was
in bounds, not on a UTF-8 char boundary, so a glyph name containing
non-ASCII bytes could cause a slice on a non-boundary index. Switch to a
checked slice that folds into the existing Option flow, and add tests.
Co-authored-by: Abimael Martell <abimaelmartell@users.noreply.github.com>
