Add lesson 4

- Add initial `PostNodeRepository`
- Move logic to Repository
- Inject `EntityTypeManagerInterface`
- Add `PostNodeRepositoryTest`
- Add failing test method
- Failing test: non-existent service
- The "node" entity type does not exist.
- Create posts
- Add assertions based on the `created` date
- Sort nodes before returning them
- Return values to reset array keys

web/modules/custom/example/example.services.yml

@@ -0,0 +1,4 @@
+services:
+  Drupal\example\Repository\PostNodeRepository:
+    arguments:
+      - '@entity_type.manager'