Provide API for date utility functions

[justin-schroeder]

Reproduction

https://formkit.link/ba56ab8e550cd5204bbb36660d27c908

Describe the bug

Issues like #844 are helpful to highlight the difficulties of working with dates. It’s very easy, as that example shows, to provide a date to a datepicker that is immediately mutated. It would be helpful if we exported and open-sourced the date utility functions that are at work behind the scenes of the datepicker so users can provide well formatted dates and generally make date work easier.

Environment

• OS: macOS • Browser: All • Version: 0.17.5