feat(parser): add GridShow and ShowCustomThumbnail

Closes #459

src/parser/classes/ShowCustomThumbnail.ts

@@ -0,0 +1,14 @@
+import { YTNode } from '../helpers.js';
+import { RawNode } from '../index.js';
+import Thumbnail from './misc/Thumbnail.js';
+
+export default class ShowCustomThumbnail extends YTNode {
+  static type = 'ShowCustomThumbnail';
+
+  thumbnail: Thumbnail[];
+
+  constructor(data: RawNode) {
+    super();
+    this.thumbnail = Thumbnail.fromResponse(data.thumbnail);
+  }
+}