Adding support for mappers being nested under interfaces or records

[david-driscoll]

Allow mappers to be nested in records or interfaces

Description

This is a minor update to allow mappers to be nested under an interface or record in addition to class.

Checklist

• The existing code style is followed
• The commit message follows our guidelines
• Performed a self-review of my code
• Hard-to-understand areas of my code are commented
• The documentation is updated (as applicable)
• Unit tests are added/updated
• Integration tests are added/updated (as applicable, especially if feature/bug depends on roslyn or framework version in use)

[latonz]

Thank you for this great contribution!

The failing test should be fixable by simply adding a pseudo field like private readonly string _test = test;

[latonz]

As the integration tests run on all supported .NET versions the primary constructor test needs to be excluded from .NET versions < 8.0.
But IMO these tests would fit better as unit tests instead of integration tests since the generated code is not really .NET version dependent and there is already one integration test for this scenario (NestedTestMapper).

[david-driscoll]

I added the primary constructor because I've run into issues with it in the past on my own source generators, so I just thought I'd add it for completeness.

I'm working on the changes now.

[david-driscoll]

Looks like everything is passing, let me know if there are any other changes!

[latonz]

LGTM, thanks for this contribution!