Add devcontainer and Github Codespaces setup

[cleverer]

Just works ;) (If you don't know how, check the docs)

After merging it would make sense to set up prebuilds: https://docs.github.com/en/codespaces/prebuilding-your-codespaces/configuring-prebuilds

I'd recommend to run weekly rebuilds I guess.

Prebuilds run on Github Actions which is free for public repositories, so no additional costs. Only storage for the prebuilt images is not enough with 15GB…