Geometry Nodes: use index field implicitly in Set ID node

That makes the node more useful by default.
One use case is to delete some points after the Set ID node, then instance
with some randomness. Now when deleting different points, the
randomness will remain stable.

source/blender/nodes/geometry/nodes/node_geo_set_id.cc

@@ -22,7 +22,7 @@ static void geo_node_set_id_declare(NodeDeclarationBuilder &b)
 {
   b.add_input<decl::Geometry>("Geometry");
   b.add_input<decl::Bool>("Selection").default_value(true).hide_value().supports_field();
-  b.add_input<decl::Int>("ID").supports_field();
+  b.add_input<decl::Int>("ID").implicit_field();
   b.add_output<decl::Geometry>("Geometry");
 }