Obtains an iterator pointing to
char_offset within the given line.
Note characters, not bytes; UTF-8 may encode one character as multiple bytes.
line_number is greater than or equal to 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
whether the exact position has been found