Implement YearMonth

[dkhalanskyjb]

Fixes #168
Fixes #184

Related PRs:

• After Local date range #189, we can implement YearMonth.days: LocalDateRange and ranges of YearMonth values.
• Without Implement the same ranges for all platforms #453, YearMonth.until returning Long is inconsistent with LocalDate.until: Int. Also, this PR removes the js-joda-based JS implementation, which makes it easier to make YearMonth an expect class if we want that.
• Rework the serializers using the DateTimeFormat API #415 affects which serializers are available and what they are called.
• Replace dayOfMonth with day and monthNumber with month #387 proposes to remove monthNumber and rename the monthNumber: Int constructor parameter to month: Int, which affects YearMonth as well.
• Implement java.io.Serializable for some of the classes #373 could also implement java.io.Serializable for YearMonth.