Discontinue this library once Xunit ships its new support for skipping

[AArnott]

Xunit itself recently acquired skippable test support: xunit/xunit#2073

Once that ships on nuget.org, we'll shut this repo down.

[irontoby]

Thanks for calling this out! I'm guessing "official" support will only appear in xUnit v3?

This package has been a big help in our integration tests, and I was looking at extending it to support our specific requirements but based on this it seems it's best to wait until v3 progresses from alpha.

[AArnott]

@irontoby Feel free to send issues (and especially PRs) if you're still using xunit v2 and want more functionality. My time is limited but if you are willing to send PRs (preferably after your proposal is ratified via a github issue) I'm happy to keep merging in new functionality.

[perlun]

In case this helps someone like me: this library is still useful, while we wait for xUnit v3. xunit/xunit#2073 (comment)

(What particularly confuses me is that xUnit v2 Xunit.Sdk.SkipException, but throwing it doesn't properly skip a test...? I guess it's just an internal exception used to implement [Fact(Skip = "...")] in the library or something.)