Implement YearMonth

[dkhalanskyjb]

Fixes #168 Fixes #184

Related PRs:

• After https://github.com/Kotlin/kotlinx-datetime/pull/189, we can implement YearMonth.days: LocalDateRange and ranges of YearMonth values.
• Without https://github.com/Kotlin/kotlinx-datetime/pull/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.
• https://github.com/Kotlin/kotlinx-datetime/pull/415 affects which serializers are available and what they are called.
• https://github.com/Kotlin/kotlinx-datetime/pull/387 proposes to remove monthNumber and rename the monthNumber: Int constructor parameter to month: Int, which affects YearMonth as well.
• https://github.com/Kotlin/kotlinx-datetime/pull/373 could also implement java.io.Serializable for YearMonth.