Obtains an iterator pointing to
char_offset within the given line.
Note characters, not bytes; UTF-8 may encode one character as multiple bytes.
Before the 3.20 version, it was not allowed to pass an invalid location.
Since the 3.20 version, if
line_number is greater than the number of lines in the this, 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.
iterator to initialize
line number counting from 0
char offset from start of line