I found the problem with my enqueue statements. There was a space ahead of the handle name of the wp_enqueue_scripts statements for both “tablesorterjs” and “tablesorterfunctions”. Once I removed the space in front of the handle name, the scripts enqueued.
wp_register_script( 'tablesorterjs', plugins_url( '/js/jquery.tablesorter.js', __FILE__ ), array('jquery') );
wp_enqueue_script( 'tablesorterjs' );
wp_register_script( 'tablesorterfunctions', plugins_url( '/js/tablesorter.js', __FILE__ ), array('jquery') );
wp_enqueue_script( 'tablesorterfunctions' );