Add test and implementation for duplication op
Basic check for empty stack

Check stack is not empty before a pop, throw an error otherwise
Add STACK system command
Added division
Added test for multiplication and addition in the same operation
Dispatch functions for conditions and comments
Implement integer comparison operations
Define Forth conditions
Define Forth comments
Implement VERSION
Fixed EXIT command
Add Credits for Simon

Proper credits
Define COMMANDS
Dispatch for Forth functions
Forth functions definition
Define words NOP and DROP