Add support for NUnit version 4.

[manfred-brands]

Detect ClassicAssert

Fixes #610
Fixes #606
Fixes #562
Fixes #617
Fixes #618
Fixes #620

[manfred-brands]

@mikkelbu I have tested this extensively with my work projects and converted about 50,000 classic asserts, including StringAssert and CollectionAssert. In addition it converted string format-specification+params into formattable strings.

I didn't make separate PRs as some build on others (params -> interpolatable).

We can release this as 3.90.
For 4.0 we then drop the netstandard1.6 build.

[mikkelbu]

Great work @manfred-brands 👍 . I've reviewed the majority of the changes, but I still need to review 21 files (mostly the new analyzers and code-fixes). I should have time for this tomorrow.

So far I've only some minor comments and questions.

[manfred-brands]

I have added the two mentioned changes and rebased on latest master.

[mikkelbu]

Tremendous work @manfred-brands 👍. I found CodeFixHelper.cs a little hard to follow, so perhaps we need some more tests of this (we can also postpone this - just so we can release all the improvements as quickly as possible).

I'm happy to get this in (and release it) when you are

[manfred-brands]

@mikkelbu There are some discussions about renaming the ClassicAsserts which would mean changing this.
If you want to release it now, it probably should be a beta package.