Add devcontainer for reproducible dev environment

This adds a .devcontainer/ folder, which is a combination of a docker file, a VSCode configuration file, and a script, that together can be used automatically by VSCode to set up a dev environment, perfectly reproducibly.

To use:

1. Install Docker.
2. Install the Remote Development extension pack.
3. Checkout this branch
4. In VSCode, open the ligolang folder, then click the "Reopen in container" button that appears in the lower right.
5. Open the integrated VSCode terminal (if it didn't open automatically) and run dune build!

It's set up to automate everything needed to start developing, including installing the vendor dependencies automatically and setting up editor IDE features. [edit: it doesn't actually set up ide features, this mr is needed for that]