Adds flutter 3.0.0 support for now (#810 An error occurred )

lib/src/widgets/raw_editor.dart

@@ -1171,6 +1171,16 @@ class RawEditorState extends EditorState
     PasteTextIntent: _makeOverridable(CallbackAction<PasteTextIntent>(
         onInvoke: (intent) => pasteText(intent.cause))),
   };
+
+  @override
+  void insertTextPlaceholder(Size size) {
+    // TODO: implement insertTextPlaceholder
+  }
+
+  @override
+  void removeTextPlaceholder() {
+    // TODO: implement removeTextPlaceholder
+  }
 }
 
 class _Editor extends MultiChildRenderObjectWidget {