Add generics to ContainerInterface

[nicolas-grekas]

WDYT?

[mnapoli]

Duplicates #35?

[nicolas-grekas]

Quite close indeed. I'd just not add generics to the keys. Despite #35 being closed, I still think this would be a good idea. Locators scoped to some interface are quite common. It'd be great to hint autocompletion about them:
ContainerInterface<Foo> $container->get('bar')->... and be suggested with what Foo provides.

The fact that psalm/phpstan would complain about this shouldn't be a blocker to me. Eg they could stop complaining when the template type is of mixed.