Extend the parser

Extend the parser, to support division and subtraction.

TODO

1. Add tests for division and subtraction.
2. Add support for subtraction, by doing the following:
   1. Modify the data structures for tokens and the AST.
   2. Add support for subtraction to the lexer.
   3. Add support for subtraction to the parser.
   4. Add support for subtraction to the evaluator.
   5. Run the tests.
3. Add support for division, following the same steps you used to add support for subtraction. In the evaluator, you do not need to worry about division by 0.