Enable modern/cross-platform development

[mattleibow]

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

Problem

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

Solution

• Enable develpers to dotnet build and dotnet test everything.
• Enabled end users/consumers to dotnet tool install mdoc or dnx mdoc

Tasks

Here are some task that are not in any particular order:

• Add CI
  • Add CI for current state: Add initial macOS CI #738
  • Add CI for Windows tests and all builds: Add Windows tests to CI #739
  • Add CI for Linux builds: Add Linux build to CI #740
• Modernize testing
  • Move test projects and helpers to a tests/ folder Move the tests out of the source directory and add CI #736
  • Migrate to dotnet test for NUnit tests (Instead of mono and the console runner)
  • Migrate away from Makefiles and into NUnit test methods
• Create .NET CLI tool
  • Update the mdoc project to pack as a tool
  • Ensure .NET Full Framework tool is packed
  • Ensure best backwards compatibility
• Update dependencies
  • Update to a supported .NET SDK (.NET 6 is out of support)
• Enable modern AI development
  • Add a copilot-setup-steps.yml workflow
  • Add a copilot-instructions.md file

[mattleibow]

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