DEV: Add a new type_source field to the Reviewable model. (#31325)

This change adds a new `type_source` field to the `Reviewable` model, indicating whether the Reviewable type was registered by `core`, a plugin, or an `unknown` source.

When a plugin that registered a Reviewable type is disabled, this allows us to tell the user which plugin they need to re-enable to handle any orphan reviewable items.

spec/lib/plugin/instance_spec.rb

@@ -625,13 +625,22 @@ TEXT
     subject(:register_reviewable_type) { plugin_instance.register_reviewable_type(new_type) }
 
     context "when the provided class inherits from `Reviewable`" do
-      let(:new_type) { Class.new(Reviewable) }
+      let(:new_type) do
+        class MyReviewable < Reviewable
+        end
+        MyReviewable
+      end
 
       it "adds the provided class to the existing types" do
         expect { register_reviewable_type }.to change { Reviewable.types.size }.by(1)
         expect(Reviewable.types).to include(new_type)
       end
 
+      it "shows the correct source for the new type" do
+        register_reviewable_type
+        expect(Reviewable.source_for(new_type)).to eq("discourse-sample-plugin")
+      end
+
       context "when the plugin is disabled" do
         before do
           register_reviewable_type