use github actions for CI and add compatibility to php 8

make validator and filter invokable

execute `Field::filter()` in `Field::validate()`

This ensures that the value given is filtered before it get's validated.
Beside that `Field::filter()` gets now executed twice what is acceptable
because filters caches (when possible) the filtered value.

test only for integer when string is numeric

`(double)'foo'` always returns 0 what is an integer too.

/solves #17

Unfortunately case is a keyword and we can not use it as class name.

We need this for example when we don't want a specific card type:
`['creditCard', '!credictCard:amex:dinersclub']`