Add a dev container environment

[qobilidop]

I personally found dev containers pretty convenient to use. So maybe it's useful to set up one for this repo.

[qobilidop]

The same dev container configuration also enables GitHub Codespaces. But I'm not sure how the billing works when using it in an org repo. So I didn't mention it in the README.

An alternative I could think of is to suggest users fork this repo and use Codespaces from their fork if they want. Every GitHub personal account has some free Codespaces quotas per month. So this might be a solution.