Updating repo for transfer to the .NET Foundation

[tannergooding]

This updates the repo so it can finish transfer to the .NET Foundation: dotnet-foundation/projects#184

In particular, now that the project agreement has been signed, this:

• Updates the copyright to be .NET Foundation and Contributors rather than Microsoft and Contributors
• Updates the license from NCSA to MIT
  • All prior contributions to LLVMSharp were made under the Microsoft CLA
  • NCSA was originally chosen to align with LLVM
  • LLVM have themselves since changed to Apache 2.0 License with LLVM exceptions
  • The libLLVM nuget package is remaining Apache 2.0 License with LLVM exceptions since that is a binary produced by LLVM themselves
• Updates the code of conduct to be the .NET Foundation Code of Conduct rather than the Microsoft Open Source Code of Conduct

URLs and CI are still microsoft/ClangSharp and will need to be updated after the repo moves out of the microsoft GitHub organization.

[tannergooding]

CC. @ChrisSfanos