Add Funcky.Xunit Packages for xUnit v3

[tautropfli]

xUnit v3 is finally here 🎉

We could either:

• Update Funcky.Xunit to xUnit v3. This means that we force ourselves and our consumers to update to xUnit v3 right now (a lot of third-party libraries such as FsCheck.Xunit are not yet updated to v3)
• Take the same approach that xUnit itself uses i.e. appending .v3 to the packages. This is what this PR does.

Do we still want to include ToTheoryData in v3? The new theory data is much more powerful:

• Collection expressions work for TheoryData:

  public static TheoryData<int, string> IntegersAndStringRepr => [
      (10, "10"),
      (20, "20"),
  ];

• Enumerables can easily be converted to TheoryData by passing it to its constructor:

  public static TheoryData<int, string> IntegersAndStringRepr => new(
      Enumerable.Range(0, count: 5)
          .Select(n => n * 10)
          .Select(n => (n, n.ToString())));

[FreeApophis]

I think you can remove it from v3. Otherwise I think that’s the only nice way, even though it is very stupid..