Change Temporal.Calendar.prototype.toJSON not to call toString

Normative PR: https://github.com/tc39/proposal-temporal/pull/2482

test/built-ins/Temporal/Calendar/prototype/toJSON/returns-identifier-slot.js

@@ -0,0 +1,23 @@
+// Copyright (C) 2023 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.calendar.prototype.tojson
+description: toJSON() returns the internal slot value
+includes: [compareArray.js, temporalHelpers.js]
+features: [Temporal]
+---*/
+
+const actual = [];
+
+const calendar = new Temporal.Calendar("iso8601");
+TemporalHelpers.observeProperty(actual, calendar, Symbol.toPrimitive, undefined);
+TemporalHelpers.observeProperty(actual, calendar, "id", "bogus");
+TemporalHelpers.observeProperty(actual, calendar, "toString", function () {
+  actual.push("call calendar.toString");
+  return "gregory";
+});
+
+const result = calendar.toJSON();
+assert.sameValue(result, "iso8601", "toJSON gets the internal slot value");
+assert.compareArray(actual, [], "should not invoke any observable operations");