JSON converter for YearMonth type

[skarllot]

The NodaTime library currently lacks a JSON converter for the YearMonth type, which prevents proper serialization and deserialization when working with JSON.

Current Situation

• JSON converters exist for other NodaTime types (such as LocalDate, LocalDateTime, etc.)
• When attempting to serialize/deserialize YearMonth objects, users currently need to implement custom solutions

Proposed Solution

I would like to implement a YearMonthJsonConverter that follows the same patterns and conventions as the existing NodaTime JSON converters.

Questions

• Are there any specific requirements or patterns I should follow for consistency with the existing converters?
• Would this contribution be welcome?
• Is anyone already working on this feature?

[jskeet]

I have no objection to this - seems like a reasonable thing to do, thanks for the offer!

Hopefully the patterns are fairly obvious in the existing code - I don't expect this will be particularly hard (as YearMonthPattern already exists in NodaTime itself).

I would personally suggest restricting this to ISO calendar YearMonth values.

Please implement in both the Json.NET and System.Text.Json projects, for consistency.