Include slot-scope in attributes-order (#440)

Fixes #432

Author: GufNZ

lib/rules/attributes-order.js

@@ -29,7 +29,7 @@ function getAttributeType (name, isDirective) {
       return 'DEFINITION'
     } else if (name === 'id') {
       return 'GLOBAL'
-    } else if (name === 'ref' || name === 'key' || name === 'slot') {
+    } else if (name === 'ref' || name === 'key' || name === 'slot' || name === 'slot-scope') {
       return 'UNIQUE'
     } else {
       return 'OTHER_ATTR'