Add week number and year properties

[erikc5000]

This adds support for weekOfMonth, weekOfYear, weekBasedYear, weekOfWeekBasedYear, and lengthOfWeekBasedYear. Debated using weekYear instead of weekBasedYear, but it's likely that a YearWeek class will be introduced at some point, so that might get confusing.

Improved the code generation alongside this to make synchronized changes across the different date-time classes easier.