instead of inside implementation configuration.
Whether a field is a child element is a core concept of the tree,
not a matter of particular implementation class.
It is especially visible in IR tree, where the base classes implement the
acceptChildren method. It will also be more relevant in the future.
This also aligns the config style between IR and FIR tree generators.
^KT-65773 In Progress
Wojciech Litewka
e737320d01