Small RBI file updates to compensate for missing constants

Author: paracycle

sorbet/rbi/shims/erb.rbi

@@ -0,0 +1,3 @@
+# typed: true
+
+module ERB::Escape; end

sorbet/rbi/shims/set.rbi

@@ -4,3 +4,6 @@ class Set
   sig { returns(T.self_type) }
   def compare_by_identity; end
 end
+
+module Set::SubclassCompatible; end
+module Set::SubclassCompatible::ClassMethods; end