Use ReadonlyArray for date picker model type

[G-Rath]

Currently the default and model values for the DatePicker component are Date | Array<Date> | Array<Date | null> | undefined | null even though the component will never modify the array itself, so it could be typed as ReadonlyArray.

(this should be safe as due to how v-model is expected to work, the component should always be emitting a new array rather than modifying the existing array)

This would be nice as it would allow me to pass in my readonly arrays without having to do any casting

[G-Rath]

I'm happy to look into doing a PR for this, but the previous PR I opened got closed without being merged due to internal processes 🙃