feat: word counter service (#705)

* feat: word counter service

* chore: minor improvement

* test: add expect for stop()

* fix: code review

test/plugins/word_count/word_counter_test.dart

@@ -0,0 +1,67 @@
+import 'package:appflowy_editor/appflowy_editor.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+import '../../new/infra/testable_editor.dart';
+
+void main() async {
+  late final WordCountService service;
+
+  setUpAll(() {
+    TestWidgetsFlutterBinding.ensureInitialized();
+  });
+
+  tearDown(() {
+    service.dispose();
+  });
+
+  group('word_counter_service.dart', () {
+    testWidgets(
+      'Word and Character count updates',
+      (tester) async {
+        const text = 'Welcome to Appflowy!';
+        final editor = tester.editor..addParagraphs(3, initialText: text);
+        await editor.startTesting();
+
+        service = WordCountService(editorState: editor.editorState)..register();
+
+        expect(service.wordCount, 3 * 3); // 9 Words
+        expect(service.charCount, text.length * 3);
+
+        int wordCount = 0;
+        int charCount = 0;
+
+        void setCounters() {
+          wordCount = service.wordCount;
+          charCount = service.charCount;
+        }
+
+        service.addListener(setCounters);
+
+        final transaction = editor.editorState.transaction
+          ..insertText(
+            editor.editorState.getNodeAtPath([0])!,
+            text.length,
+            text,
+          );
+
+        await editor.editorState.apply(transaction);
+
+        await tester.pumpAndSettle();
+
+        expect(service.wordCount, 3 * 4);
+        expect(service.charCount, text.length * 4);
+        expect(wordCount, 3 * 4);
+        expect(charCount, text.length * 4);
+
+        service.stop();
+
+        expect(service.wordCount, 0);
+        expect(service.charCount, 0);
+        expect(wordCount, 0);
+        expect(charCount, 0);
+
+        service.removeListener(setCounters);
+      },
+    );
+  });
+}