Add new Date field widget to utilize HTML5 #date input type

Follow-up issue for https://github.com/backdrop/backdrop-issues/issues/3972

Now that we have html5 date fields in core, we should also provide them in core date module. Possibly even replace the jQuery-based datepicker completely (not sure yet, if that's actually possible).

At least it should be possible for admins / site builders to provide consistent date field widgets for node (entity) forms.