Support for xunit.v3

[ljb]

Recently, a new major version of xUnit was released. See:
https://xunit.net/docs/getting-started/v3/migration

It would be nice if support was added for this to ArchUnitNET.

I just finished migrating a large code base to use xunit.v3, and ArchUnitNET is the only of my dependencies that doesn't support it.

The new major version of xUnit uses a new NuGet package name: xunit.v3. I have seen that different libraries handle this situation in different ways. Some libraries released a new major version, like FsUnit did with version 7.0.0. Some libraries have opted for creating a new package name. One example of the latter is Verify where they have created a new package name called Verify.XunitV3.

I'm not sure which approach fits ArchUnitNET best.

[ljb]

I created a pull request for this:
#339

I added a new project called ArchUnitNET.xUnitV3. This follows the same naming convention that is used for ArchUnitNET.MSTestV2. Having a separate project allows having support both for xUnit 2 and xUnit 3.

The new project is basically a copy of the ArchUnitNET.xUnit, but uses xunit.v3.assert for the assertions. I think using copy-and-paste is preferable in this case since the project only consists of three small C# files.