Add Pattern entity

Closes #3479643

config/schema/experience_builder.schema.yml

@@ -243,3 +243,38 @@ field.field_settings.component_tree:
   type: field.field_settings.*
   constraints:
     FullyValidatable: ~
+
+experience_builder.pattern.*:
+  type: config_entity
+  constraints:
+    FullyValidatable: ~
+  mapping:
+    id:
+      type: machine_name
+      label: 'ID'
+    label:
+      type: required_label
+      label: 'Label'
+    # A single component tree.
+    component_tree:
+      constraints:
+        ComponentTreeMeetRequirements:
+          # Only StaticPropSources may be used, because a Pattern cannot rely on an entity of a particular type
+          # and bundle to be present everywhere it is inserted/used.
+          props:
+            absence:
+              - dynamic
+              - adapter
+            presence: ~
+          tree:
+            absence:
+              # Components implementing either of these 3 interfaces are only
+              # allowed to live at the PageTemplate level.
+              # @see \Drupal\experience_builder\Entity\PageTemplate
+              # @see `type: experience_builder.page_template.*`
+              - Drupal\Core\Block\MainContentBlockPluginInterface
+              - Drupal\Core\Block\TitleBlockPluginInterface
+              - Drupal\Core\Block\MessagesBlockPluginInterface
+            presence: ~
+      type: experience_builder.component_tree
+      label: 'The component tree that represents a reusable pattern'