Feat: re-add time data to output dates.

[ItsMeBrianD]

Resolves #83.

Simple 1-line fix, assigns the previous states time data to the new state before replacing it, thus ensuring it will remain available.

[6eDesign]

We will need to re-think how we want to do this in v3 of svelte-calendar. I do agree with the basic premise: svelte-calendar's Datepicker component should not be altering the time portion of the selected dates for implementations like yours. I have worked around this with my own time picker implementations but it would be nice if workarounds were not necessary.