feat: implement remove subtype if text node is empty when pressing enter key

frontend/app_flowy/packages/flowy_editor/lib/service/internal_key_event_handlers/enter_in_edge_of_text_node_handler.dart

@@ -29,6 +29,25 @@ FlowyKeyEventHandler enterInEdgeOfTextNodeHandler = (editorState, event) {
 
   final textNode = nodes.first as TextNode;
   if (textNode.selectable!.end() == selection.end) {
+    if (textNode.subtype != null && textNode.delta.length == 0) {
+      TransactionBuilder(editorState)
+        ..deleteNode(textNode)
+        ..insertNode(
+          textNode.path,
+          textNode.copyWith(
+            children: LinkedList(),
+            delta: Delta([TextInsert('')]),
+            attributes: {},
+          ),
+        )
+        ..afterSelection = Selection.collapsed(
+          Position(
+            path: textNode.path,
+            offset: 0,
+          ),
+        )
+        ..commit();
+    } else {
       final needCopyAttributes = StyleKey.globalStyleKeys
           .where((key) => key != StyleKey.heading)
           .contains(textNode.subtype);
@@ -48,6 +67,8 @@ FlowyKeyEventHandler enterInEdgeOfTextNodeHandler = (editorState, event) {
           ),
         )
         ..commit();
+    }
+
     return KeyEventResult.handled;
   } else if (textNode.selectable!.start() == selection.start) {
     TransactionBuilder(editorState)