Write modifiers in grammar instead of overloads

[Felienne]

In some cases, esp in higher levels it would be more elegant and more flexible if we would modify a rule rather than override is.
f.e. in level 9 when we add : to the if, we now write this:

ifs: _IF _SPACE condition _COLON _EOL (_SPACE command) (_EOL _SPACE command)* _EOL _END_BLOCK

Something that would modify the if to insert _COLON would be a lot nicer! Not sure what that would look like but still cool.

[Felienne]

Fixed by #1311