getIterAtLineOffset
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.
Return
whether the exact position has been found
Parameters
iter
iterator to initialize
lineNumber
line number counting from 0
charOffset
char offset from start of line