This implements a new builtin node tree type called SimulationNodeTree.
It is not yet embedded in the Simulation data block.
This is part of T73324.
The WITH_NEW_SIMULATION_TYPE cmake option is used to control whether Simulation Editor is shown in the editors menu.
Disabling the rna code with this option was a bit tricky, because the node tree type stores a reference to the rna type.
I could do the #ifdef WITH_NEW_SIMULATION_TYPE everywhere, but I'm not sure if it is worth the effort.
Currently there is only the group node, which is unusable, because there are no other nodes.
I'll submit those for review separately.