Add deprecated attribute to element serializer

Alchemy 5.2 introduced a new feature for the element definition.
Since the definition is just a hash we can safely use the attribute
on older versions as well.

app/serializers/alchemy/json_api/element_serializer.rb

@@ -11,6 +11,11 @@ class ElementSerializer
         :created_at,
         :updated_at,
       )
+
+      attribute :deprecated do |element|
+        !!definition["deprecated"]
+      end
+
       belongs_to :parent_element, record_type: :element, serializer: self
 
       belongs_to :page, record_type: :page, serializer: ::Alchemy::JsonApi::PageSerializer

spec/serializers/alchemy/json_api/element_serializer_spec.rb

@@ -27,6 +27,7 @@
       expect(subject[:created_at]).to eq(element.created_at)
       expect(subject[:updated_at]).to eq(element.updated_at)
       expect(subject[:position]).to eq(element.position)
+      expect(subject[:deprecated]).to eq(false)
       expect(subject.keys).not_to include(:tag_list, :display_name)
     end