Add generics to AbstractRector, RectorInterface and ConfigurableRectorInterface

[mspirkov]

I’m developing an extension for Rector and I’ve come across some PHPStan errors:

I propose solving this as follows:

1. Add generics to AbstractRector, RectorInterface, and ConfigurableRectorInterface.
2. Specify the return type as list for getNodeTypes, since it is essentially always list.

This will also make the code more concise and clear, in my opinion.

[mspirkov]

I see that rector-laravel also has this error, but it’s simply ignored: https://github.com/driftingly/rector-laravel/blob/2636a2af80ecb35fff8dd1e5d67ff939becb2673/phpstan.neon#L23