Improve devcontainers

[lucyb]

References https://github.com/opensafely-core/codespaces-initiative/issues/43 and https://github.com/opensafely-core/codespaces-initiative/issues/9

This will provide a development environment for researchers containing:

• Python 3.10
• The equivalent Python packages based on the Python Action image, including ehrql
• R 4.0.5
• RStudio
• The equivalent R packages based on the R Action image

This can be used in Codespaces or locally via devcontainers in VS Code.

Still to do:

• [x] Set up the Python virtual env correctly (so we get ehrql autocomplete at a minimum)
• [x] Set up the renv correctly so the packages can be accessed from within the R environment
• [ ] Remove our reliance on Tom P's image
• [ ] Build the image ourselves and uploading to ghcr rather than building each time (EDIT I've manually uploaded an image for the purposes of testing)