Create Unit Test extension update for Xunit

[AbhitejJohn]

• Ensured that this installs on the latest VS.
• Validate Create Unit Test support with .Net Core.
• Added a provider for the latest xunit Fx and Adapter versions and removed those for earlier less used versions.
• Simplified project creation logic.
• Using the base implementation to determine IsTestProject checks similar to Nunit. We could split this up if we need to in the future - At the moment I dont see a need.

[AbhitejJohn]

Tagging @bradwilson and @onovotny to help with review.

[AbhitejJohn]

@bradwilson , @Onovotny: Just pinging back on this again. It'd be great to have xunit supported in the Create Unit Test workflow again.