YearWeek, YearMonth and Year are missing `public static parse(string $iso): self`

[gnutix]

Hello there,

I've noticed that YearWeek, YearMonth and Year each have a __toString() method returning an ISO representation of the object, but none have a public static function parse(string $iso): self method. Which leads to multiple usages of *::of() methods sprouting around in our code (some using substr, some explode('-W'), etc... unpredictable code, no proper error management... nasty things).

Is that something you would be willing to accept as a PR?

Thanks for your answer. gnutix