refactor: turn getValueAsDate/setValueAsDate into a get/set property

[DavideMininni-Fincons]

Convert getValueAsDate/setValueAsDate into a get/set property valueAsDate (sbb-time-input and sbb-datepicker). Proposal: use a couple of get/set properties (value, valueAsDate) just like on native input elements in order to avoid ambiguous scenarios like timeInput.valueAsDate = “2024-10-10T09:00:00.000Z”.