Add support for use by the .NET Core runner

[CharliePoole]

Package will need to include either a .NET Standard or .NET Core build.

[CharliePoole]

There are two ways to handle this:

1. First fix Multi-targeted Engine Extensions nunit-console#394, which may require a bit of work.
2. Provide an adhoc fix, by creating a second, separate package NUnit.Extension.NetStandardProjectLoader.

For 4.X, I think option 1 is preferable, for 3.18.0, I think either one is good but option 1 does avoid the problem of proliferating extensions we would probably end up deprecating in V4.

Although Chris and I spent a lot of time discussing the options for nunit/nunit-console#394, I need to review the code in detail to decide how difficult a change it is. If it's not too bad, that would be my preference.

@OsirisTerje Any thoughts on this? Since both the netcore runner and the adapter run tests in the existing process, I'm assuming that what works for one will work for the other.

[CharliePoole]

I've gone with option 1, fixing nunit/nunit-console#394. The nunit project loader extension is now multi-targeted to .NET Framework 2.0 and Net Standard 2.0