New super-splice operator

[stlutz]

In Ohm v15.3.0 a new operator was added:
ohmjs/ohm@b519a05
https://github.com/harc/ohm/blob/master/doc/syntax-reference.md#defining-extending-and-overriding-rules

The super-splice operator (...) can be used to append and/or prepend cases to the supergrammar rule body. E.g., if the supergrammar defines comment = multiLineComment, then comment := ... | singleLineComment is equivalent to comment := multiLineComment | singleLineComment.