undo and redo

components/wysiwyg-editor.js

@@ -54,6 +54,12 @@ class WYSIWYGEditor extends LitElement {
   render() {
     return html`
       <div class="toolbar">
+        <button @click="${() => this.execCmd('undo')}">
+          <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 512 512"><path d="M48.5 224H40c-13.3 0-24-10.7-24-24V72c0-9.7 5.8-18.5 14.8-22.2s19.3-1.7 26.2 5.2L98.6 96.6c87.6-86.5 228.7-86.2 315.8 1c87.5 87.5 87.5 229.3 0 316.8s-229.3 87.5-316.8 0c-12.5-12.5-12.5-32.8 0-45.3s32.8-12.5 45.3 0c62.5 62.5 163.8 62.5 226.3 0s62.5-163.8 0-226.3c-62.2-62.2-162.7-62.5-225.3-1L185 183c6.9 6.9 8.9 17.2 5.2 26.2s-12.5 14.8-22.2 14.8H48.5z"/></svg>
+        </button>
+        <button @click="${() => this.execCmd('redo')}">
+          <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 512 512"><path d="M463.5 224H472c13.3 0 24-10.7 24-24V72c0-9.7-5.8-18.5-14.8-22.2s-19.3-1.7-26.2 5.2L413.4 96.6c-87.6-86.5-228.7-86.2-315.8 1c-87.5 87.5-87.5 229.3 0 316.8s229.3 87.5 316.8 0c12.5-12.5 12.5-32.8 0-45.3s-32.8-12.5-45.3 0c-62.5 62.5-163.8 62.5-226.3 0s-62.5-163.8 0-226.3c62.2-62.2 162.7-62.5 225.3-1L327 183c-6.9 6.9-8.9 17.2-5.2 26.2s12.5 14.8 22.2 14.8H463.5z"/></svg>
+        </button>
         <button @click="${() => this.execCmd('bold')}">
           <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 384 512"><path d="M0 64C0 46.3 14.3 32 32 32H80 96 224c70.7 0 128 57.3 128 128c0 31.3-11.3 60.1-30 82.3c37.1 22.4 62 63.1 62 109.7c0 70.7-57.3 128-128 128H96 80 32c-17.7 0-32-14.3-32-32s14.3-32 32-32H48V256 96H32C14.3 96 0 81.7 0 64zM224 224c35.3 0 64-28.7 64-64s-28.7-64-64-64H112V224H224zM112 288V416H256c35.3 0 64-28.7 64-64s-28.7-64-64-64H224 112z"/></svg>
         </button>