Clean em dashes from ISBNs during sanitization

src/MD_STD_IN.php

@@ -411,7 +411,7 @@ public static function validate_isbn(string $input):string {
         }
 
         // Remove hyphens
-        $input = trim(strtr($input, ["-" => "", "–" => ""]));
+        $input = trim(strtr($input, ["-" => "", "–" => "", '—' => '']));
 
         // ISBN 10
         if (\mb_strlen($input) === 10) {