Added exponentiation operator

[aloshdenny]

Added support for exponentiation operator

[aloshdenny]

Compilers still don't make sense to me, but I'm not stopping until I figure it out.

[rohittp0]

PRINT "Enter first number"
INPUT a
PRINT "Enter second number"
INPUT b

LET c = a**b

PRINT c

Invalid token 'Token(TokenType.EXP:'**')'