Spring Add a devcontainer for VSCode and Github Codespaces

With this change user can start a container locally with VSCode remote, or open an IDE from Github in the browser.

VSCode doesn't support .aj sources yet, so it's not a completely smooth experience, but you can make changes and build from the command line.

Comment From: snicoll

Thanks for the PR @dsyer but we're not confident in our ability to maintain this in the long run. We don't use a cloud setup on a day-to-day basis, so only newcomers are likely to find out about issues.