feat(ytmusic): add support for artists

Available through `Innertube#music.getArtist(id: string)`

#78

lib/parser/contents/classes/MusicPlaylistShelf.js

@@ -0,0 +1,22 @@
+'use strict';
+
+const Parser = require('..');
+
+class MusicPlaylistShelf {
+  type = 'MusicPlaylistShelf';
+  
+  #continuations;
+  
+  constructor(data) {
+    this.playlist_id = data.playlistId;
+    this.contents = Parser.parse(data.contents);
+    this.collapsed_item_count = data.collapsedItemCount;
+    this.#continuations = data.continuations;
+  }
+  
+  get continuation() {
+    return this.#continuations?.[0]?.nextContinuationData;
+  }
+}
+
+module.exports = MusicPlaylistShelf;