Move some core type into internal & migrate to slnx

[Romfos]

Changes:

• Move some core type into internal
• Migrate to *.slnx solution file (see https://devblogs.microsoft.com/dotnet/introducing-slnx-support-dotnet-cli/)
• Update readme to show build status for main branch

This PR is a follow up for discussion #869 and alternaltive solution when we don't what make these types "true" internal\sealed but still want to logically separate these types

Core types that are still "public" core namespace:

1. Interfaces
2. Container, Context and di stuff
3. Some core types that were referenced in tests (goal was to make NSubstitute.Acceptance.Specs to have 0 references to internal namespace)

Related #830

[304NotModified]

There are multiple internal types leaking into the public API,
See #881

I do think we should keep internal types, internal. Do you agree @Romfos ?

Update: I think most of the "violations" could be easily fixed with an public interface

[304NotModified]

Is this some other change, as it isn't the internal namespace?
Are more namespaces changed in this PR?

[dtchepak]

Initially the NSubstitute.Core namespace was meant to be internal/use-at-your-own-risk. NSubstitute was for the public API. Is the NSubstitute.Internal change just to make this more specific?

The MR looks good, will approve pending feedback on @304NotModified's comments. 👍

[304NotModified]

@dtchepak
What's a MR? Do you mean PR?

[dtchepak]

@dtchepak What's a MR? Do you mean PR?

Haha, yes, sorry. I do a lot with gitlab where they're known as Merge Requests.

[Romfos]

@304NotModified thank you for your feedback.
the more I go deep to unit test that @304NotModified mention then more I understand how crazy this feature will be

I'm afraid that this approach will make more problems then solve
Original idea was to simplify future changes and make support easier

Lets save status quo for now and don't do anything with it