Add remote container development environment #693

I just stumbled across this awesome article: https://www.freecodecamp.org/news/put-your-dev-env-in-github/

I had heard about remote containers and VSCode's integration with Docker and WSL, but this is the first time I realized how awesome this is for open source projects.

Granted, Units.NET does not require a lot of dependencies beyond Windows and .NET Core SDK, but the idea is really compelling: Simply clone the repo and spin up a development container with everything you need to build and run the project, without having to install anything on your host operating system besides VSCode. I'm sure other IDEs can be made to work with this setup too.

If someone wants to take this for a spin and see how it works for Units.NET, please go ahead. If not, I may visit this sometime in the future mostly to scratch my own curiosity.