Add DayOfMonth type

[bdsl]

Proposing a type to to represent a specific day of the month, between 1 and 31. Similar to the existing type DayOfWeek

The immediate use case I have in mind for this is to record which day a monthly payment should happen on, but there are probably several other use cases, and of course this could be used a constituent of existing types MonthDay, LocalDate etc.

I'm not sure if would be better implemented as a class or an Enum - 31 seems near the border of being too many or not too many cases for an enum.

I might have time to implement this if its desired.