dev: create ytmusic class

In future versions anything related to YouTube Music will be implemented here. The main Innertube class will expose it to users as `Innertube#music`.

lib/core/Music.js

@@ -0,0 +1,12 @@
+// Everything related to yt music will go here
+
+/** @namespace */
+class Music {
+  /**
+   * @param {Innertube} session
+   * @constructor
+   */
+  constructor(session) {
+    // TODO: finish this
+  }
+}