feat: add class manipulation via functions

[jaulz]

This PR allows is a bit more complex but I think the changes will allow much more flexibility. Classes passed via props do not override the classes from variants by default anymore. If you pass undefined it will still clear the variant classes but for other values it will merge them. However, now you can also pass a function to the class object which allows you to manipulate the previous classes:

P.S.: the hasProperty function might lead to some type issues at the moment because it merges the object with Record<Y, unknown> and in case Y is a generic string it will basically extend all properties with & unknown.

[alfonsobries]

@jaulz do you think you can remove the prettier stuff? its not like I am against it but I prefer further discussions since we already have eslint and I don't want to use different settings on the packages, in other words we can create separate PRs for that