[New] Parse IN and IS in the expression #511

$ lpython --show-ast --new-parser examples/expr2.py
syntax error: Token 'in' is unexpected here
 --> examples/expr2.py:1:10
  |
1 | if field in ["vararg", "kwarg"]:
  |          ^^ 


Note: if any of the above error or warning messages are not clear or are lacking
context please report it to us (we consider that a bug that needs to be fixed).