OPA1 and amyotrophic lateral sclerosis: Pectolinarigenin Improves Oxidative Stress and Apoptosis in Mouse NSC-34 Motor Neuron Cell Lines Induced by C9-ALS-Associated Proline–Arginine Dipeptide Repeat Proteins by Enhancing Mitochondrial Fusion Mediated via the SIRT3/OPA1 Axis