Geometry Nodes: Handle multiple grids in the volume to mesh node

In future use cases, a volume can contain many grids that represent the
density information. In this case, it's better if the volume to mesh node
creates a mesh based on all of the grids in the volume.

This is also a benefit to share-ability, since one doesn't have to
specify the grid name in the node. Instead, in the future we can have
a way to split particular grids into separate volumes, if only one
grid should be considered.

The code changes are relatively simple:
 - Move the old volume to mesh node to the legacy folder.
 - Run the volume to mesh node on all instance geometry, like elsewhere.
 - Make the blenkernel's volume to mesh API a bit more specific.

Differential Revision: https://developer.blender.org/D12997

release/scripts/startup/nodeitems_builtins.py

@@ -760,6 +760,7 @@ geometry_node_categories = [
     ]),
     GeometryNodeCategory("GEO_VOLUME", "Volume", items=[
         NodeItem("GeometryNodeLegacyPointsToVolume", poll=geometry_nodes_legacy_poll),
+        NodeItem("GeometryNodeLegacyVolumeToMesh", poll=geometry_nodes_legacy_poll),
 
         NodeItem("GeometryNodeVolumeToMesh"),
     ]),