Obtains an iterator pointing to @byte_index within the given line.
@byte_index must be the start of a UTF-8 character. Note bytes, not
characters; UTF-8 may encode one character as multiple bytes.
If @line_number is greater than or equal to the number of lines in the @buffer,
the end iterator is returned. And if @byte_index is off the
end of the line, the iterator at the end of the line is returned.
Obtains an iterator pointing to @byte_index within the given line.
@byte_index must be the start of a UTF-8 character. Note bytes, not characters; UTF-8 may encode one character as multiple bytes.
If @line_number is greater than or equal to the number of lines in the @buffer, the end iterator is returned. And if @byte_index is off the end of the line, the iterator at the end of the line is returned.