Queries: support for (capturing) tree-queries

[KaruroChori]

Aside from has and is, we need a capturing version of has which is able to represent a query tree.
It is way more complex to handle compared to the other two, so for now I will just pretend it is of lesser priority :).

More details and PoC soon in the docs.

[KaruroChori]

We should also have support for the "inverse" is. Not checking for all nodes satisfying a is, but checking IF a node satisfies is, basically testing the query in reverse order.
This is needed to implement any form of downstream lazy schema evaluation.