vfs: add missing JSDoc @returns for mount()

Author: mcollina

lib/internal/vfs/file_system.js

@@ -220,6 +220,7 @@ class VirtualFileSystem {
   /**
    * Mounts the VFS at a specific path prefix.
    * @param {string} prefix The mount point path
+   * @returns {VirtualFileSystem} The VFS instance for chaining
    */
   mount(prefix) {
     if (this[kMounted]) {