Add template generics for PSR-11 implementations in PHPStan and Psalm

[limarkxx]

Slim v4 abstracts out DI to PSR-11, which means given a Slim/App it is unclear what its DI container is capable of.
Psalm and PHPStan both provide mechanisms to express the types generically, allowing them to ensure that you are calling the correct set method on a DI container received from getContainer

[coveralls]

coverage: 99.427% (-0.002%) from 99.429%
when pulling c7910ea on limarkxx:add-psalm
into 8388b33 on slimphp:4.x.

[coveralls]

coverage: 99.429%. remained the same
when pulling c7910ea on limarkxx:add-psalm
into 8388b33 on slimphp:4.x.

[akrabat]

Thanks!