Prevent setting optional/nullish/nullable when a default value is set

[bramvanderholst]

When a default value is set, it can no longer be optional, nullable or nullish, otherwise the default value won't actually be used.

[AndKiel]

You can have field: Int = 15 as well as field: Int! = 15 - one would allow passing explicit null, one would not. Default is just a default, the type itself is defined as nullable or not. Having a default value has no impact on the schema type.

[Code-Hex]

This PR is closed as it appears to be suspended. If you have any questions, I would appreciate it if you could submit the PR again!