Tracking issue for the `MonthDay` component

[nekevss]

Below is the functionality needed for the MonthDay component.

• [x] 10.3.3 get Temporal.PlainMonthDay.prototype.calendarId
• [x] 10.3.4 get Temporal.PlainMonthDay.prototype.monthCode
• [ ] 10.3.5 get Temporal.PlainMonthDay.prototype.day
• [ ] 10.3.6 Temporal.PlainMonthDay.prototype.with ( temporalMonthDayLike [ , options ] )
• [x] 10.3.7 Temporal.PlainMonthDay.prototype.equals ( other )
• [x] 10.3.8 Temporal.PlainMonthDay.prototype.toString ( [ options ] )
• [ ] 10.3.9 Temporal.PlainMonthDay.prototype.toLocaleString ( [ locales [ , options ] ] )
• [x] 10.3.10 Temporal.PlainMonthDay.prototype.toJSON ( )
• [x] 10.3.11 Temporal.PlainMonthDay.prototype.valueOf ( )
• [ ] 10.3.12 Temporal.PlainMonthDay.prototype.toPlainDate ( item )
• [x] ~~10.3.13 Temporal.PlainMonthDay.prototype.getISOFields ( ) ~~ REMOVED
• [x] ~~10.3.14 Temporal.PlainMonthDay.prototype.getCalendar ( ) ~~ REMOVED