Enable modern/cross-platform development

[mattleibow]

This is a parent issue for a few things that will make the developer's lives better.

Right now there is a mix of tools used, platform restrictions and then the final package is not runnable easily.

Goals:

• Ship as a dotnet cli tool
  • Update the mdoc project to pack as a tool
  • Implement logic to also pack as a full framework tool
  • Maintain the best backwards compatibility
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• Enable modern development
  • Use the nunit packages to run instead of the separate console nuget
  • Add GitHub workflows to build and run tests on PRs
  • Enable test execution on Windows
  • Move Makefile tests into a test project
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  • Update tools to be a supported dotnet (6 is out of support)
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  • Move and flatten the nested tests out into a tests folder
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  • Prepare the repo with copilot instructions and setup steps

[mattleibow]

I am working on some of these in this PR: #734