Adds division to multiplicative expression. (#2091)

* Multiplication and division have the same priority.
* A new builtin interface DivWith is added.
* In some tests expecting a compilation error (syntax error), the error
  message now says it is expecting SLASH or binary *.

explorer/ast/expression.cpp

@@ -123,6 +123,8 @@ auto ToString(Operator op) -> std::string_view {
       return "<<";
     case Operator::BitShiftRight:
       return ">>";
+    case Operator::Div:
+      return "/";
     case Operator::Neg:
     case Operator::Sub:
       return "-";