Implement strict typing + return types + property types on all our code

Problem/Motivation

For GraphQL 5.0 we want all our types as strict as possible and leverage PHP type features.

Proposed resolution

1. Enforce types in PHPCS and/or PHPStan.
2. Add declare strict types everywhere

Any further fine-tuning or changing function signatures should be done in a follow-up, to not make this too big.

Remaining tasks

Finish PR.

User interface changes

none

API changes

Interfaces and classes will change and can cause breaking changes.

Data model changes

none