Reverted changes to AssertionScope to maintain binary compatibility

[dennisdoomen]

In #925, some of the methods of AssertionScope were changed to return an IAssertionScope instead of the full class. However, this causes binary compatibility issues for libraries extending FluentAssertions. If such a library depended on a version between 5.0 and 5.5, upgrading to 5.5 would cause missing method exception. I've reverted those changes by exposing the IAssertionScope interface as explicit interface implementations.

This should fix fluentassertions/fluentassertions.json#27

[dennisdoomen]

Note: I decided not to change Continuation.Then since it would not be possible and it is very unlikely that a lot of people are using that one.

[jnyrup]

I tried yesterday to find projects on that depend on FA and looking at ~20-30 projects that looked like they could be extension libraries, none of them used Then.