Move classic asserts into its own project

[OsirisTerje]

It should subsequentially be released as its own package, but we keep it in the framework repo and solution until it is worked out.

See details of the discussion here #3325 (comment) and the following comments after that.

[OsirisTerje]

Started moving classic asserts over into its own project, see #4417

[OsirisTerje]

@jnm2 @stevenaw @manfred-brands @mikkelbu

Are the Assert.Throws part of the classic asserts? They are used all over the nunit space, and just a few places where I see the new Assert.That(xxx,Does.Throw syntax. They also contain their own code, so not just redirecting to the constraint model.
Also, there is no Analyzer trigger on the use if it (@mikkelbu @manfred-brands ), is that because it is not regarded as classic/legacy or just because the analyzer haven't got to it yet?

[manfred-brands]

@OsirisTerje Good question.

The analyzer is virtually impossible to make for the difficult cases where the return value of Assert.Throws is used. It might just be checking the Message which could be converted if we can detect it. It could check way more and in those cases there is no new equivalent I know of. Something like: Assert.That(..., Throws.InstanceOf<SomeException>().Where(ex => ex.NonStandardProperty == ...)). If we add that generic case, an analyzer rule could possible be created.