feat: add document rules (#1071)

test/core/rules/at_least_one_editable_node_rule_test.dart

@@ -0,0 +1,31 @@
+import 'package:appflowy_editor/appflowy_editor.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  test('at least one editable node rule', () async {
+    final rule = AtLeastOneEditableNodeRule();
+    final node1 = paragraphNode(text: 'Hello World!');
+    final node2 = paragraphNode(text: 'Hello World!');
+    final node3 = paragraphNode(text: 'Hello World!');
+    final document = Document(
+      root: pageNode(
+        children: [
+          node1,
+          node2,
+          node3,
+        ],
+      ),
+    );
+    final editorState = EditorState(document: document)..documentRules = [rule];
+    // delete all the nodes
+    final transaction = editorState.transaction;
+    transaction.deleteNodes([node1, node2, node3]);
+    await editorState.apply(transaction);
+
+    await Future.delayed(const Duration(milliseconds: 100));
+    final node = editorState.document.root.children[0];
+    expect(node.type, ParagraphBlockKeys.type);
+
+    editorState.dispose();
+  });
+}