get_position
Description:
Retrieves the current line number and the number of the character on that line.
Intended for use in error messages; there are no strict semantics for what constitutes the "current" line number other than "the best number we could come up with for error messages."
Parameters:
| this | |
| line_number | 
           return location for a line number, or null  | 
      
| char_number | 
           return location for a char-on-line number, or null  |