Obtains an iterator pointing to @char_offset within the given line.
Note characters, not bytes; 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 @char_offset is off the
end of the line, the iterator at the end of the line is returned.
Obtains an iterator pointing to @char_offset within the given line.
Note characters, not bytes; 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 @char_offset is off the end of the line, the iterator at the end of the line is returned.