search: structural search rule support

Created by: rvantonder

This PR adds an experimental rule: keyword that accepts comby where rules. It is not intended that rule should be a permanent keyword since it is so specific to structural search. Unfortunately, there isn't currently a way to separate this input in the UI--when we have that, this will disappear.

Similarly, our implementation doesn't allow to separate this data type cleanly. I wanted to bundle this rule information with the rest of structural search, but ran into problems where the types need to be refactored first (which is what prompted #7182 and #7183).

Example: