Macrocyclic Spermine Alkaloids from Verbascum: The (E/Z)‐Isomeric Pairs (−)‐(S)‐Verbasitrine/(−)‐(S)‐Isoverbasitrine and (+)‐(S)‐Verbametrine/(+)‐(S)‐Isoverbametrine: Isolation, Structure Elucidation, and Synthesis