Add Gregorian calendar

This is identical to the ISO8601 calendar, except for ISO week numbering
rules, which we do not currently use in Temporal.

See: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Intl/Locale/calendar

polyfill/lib/calendar.mjs

@@ -203,3 +203,12 @@ export class ISO8601 extends Calendar {
     return ES.LeapYear(GetSlot(date, ISO_YEAR));
   }
 }
+
+// According to documentation for Intl.Locale.prototype.calendar on MDN,
+// 'iso8601' calendar is equivalent to 'gregory' except for ISO 8601 week
+// numbering rules, which we do not currently use in Temporal.
+export class Gregorian extends ISO8601 {
+  constructor() {
+    super('gregory');
+  }
+}

polyfill/lib/ecmascript.mjs

@@ -6,7 +6,7 @@ const ObjectAssign = Object.assign;
 import bigInt from 'big-integer';
 
 import { GetIntrinsic } from './intrinsicclass.mjs';
-import { ISO8601 as CalendarISO8601 } from './calendar.mjs';
+import { Gregorian as CalendarGregorian, ISO8601 as CalendarISO8601 } from './calendar.mjs';
 import {
   GetSlot,
   HasSlot,
@@ -43,6 +43,7 @@ const YEAR_MIN = -271821;
 const YEAR_MAX = 275760;
 
 const BUILTIN_CALENDARS = {
+  gregory: CalendarGregorian,
   iso8601: CalendarISO8601
   // To be filled in as builtin calendars are implemented
 };