activemodel: Add ActiveModel::API (#695)

ActiveModel::API has been added since v7.0.

refs: rails/rails#43223

gems/activemodel/7.0/activemodel-7.0.rbs

@@ -1,3 +1,16 @@
+module ActiveModel
+  module API
+    extend ActiveSupport::Concern
+    include ActiveModel::AttributeAssignment
+    include ActiveModel::Validations
+    extend ActiveModel::Validations::ClassMethods
+    include ActiveModel::Conversion
+    extend ActiveModel::Conversion::ClassMethods
+
+    def persisted?: () -> bool
+  end
+end
+
 module ActiveModel
   class Error
     CALLBACKS_OPTIONS: ::Array[:if | :unless | :on | :allow_nil | :allow_blank | :strict]