Add model date casts for Laravel 10

[davidstoker]

Laravel 8 deprecated use of the $dates property on Eloquent models and Laravel 10 removed use of it entirely in favor of $casts. Add these casts to work with Laravel 10 and leave the existing $dates property in place to allow compatibility with older Laravel versions.

Closes #111

[simonmaass]

why is this not getting merged?

[BosioGerman]

Also waiting the merge

[atymic]

Can't merge this as it's out of date, will PR in separate branch now.