Date validation using EDTF spec.

[rsdoiel]

A reference implementation in RDM is https://github.com/inveniosoftware/marshmallow-utils/blob/master/marshmallow_utils/fields/edtfdatestring.py

Topic came up in relation to Kristin Briney's Nature article, https://www.nature.com/articles/d41586-025-02210-9.

It'd be nice to have something sorted out in TypeScript/JavaScript for CL-web-component use.