Add operator modulo (#1681)

Co-authored-by: m new <michael.burzan@outlook.de>
Co-authored-by: Richard Smith <richard@metafoo.co.uk>

explorer/ast/expression.cpp

@@ -81,6 +81,8 @@ auto ToString(Operator op) -> std::string_view {
       return "or";
     case Operator::Eq:
       return "==";
+    case Operator::Mod:
+      return "%";
   }
 }