feat(Field.NationalIdentityNumber): adds fnr validation

[langz]

Motivation from https://dnb-it.slack.com/archives/CMXABCHEY/p1720521252444219

This is just the first stab at potentially introducing fnr validation to the Field.NationalIdentityNumber component.

Let's not merge this until we've had a discussion about:

• Introducing the https://github.com/navikt/fnrvalidator library in our library
• Should we even add this validation?
• Should we also add validation initially, like if should we initially display the error when providing the following not-valid fnr <Field.NationalIdentityNumber value="12345678901" />?
• Add unit tests...