feat(Parser): add support for parsing subtitle for RichShelf (#805)

src/parser/classes/RichShelf.ts

@@ -9,6 +9,7 @@ export default class RichShelf extends YTNode {
   title: Text;
   contents: ObservedArray<YTNode>;
   endpoint?: NavigationEndpoint;
+  subtitle?: Text;
 
   constructor(data: RawNode) {
     super();
@@ -18,5 +19,9 @@ export default class RichShelf extends YTNode {
     if (Reflect.has(data, 'endpoint')) {
       this.endpoint = new NavigationEndpoint(data.endpoint);
     }
+
+    if (Reflect.has(data, 'subtitle')) {
+      this.subtitle = new Text(data.subtitle);
+    }
   }
 }