devcontainer improvements

[notatallshaw]

I've been playing around with the devcontainer, I got it actually working in #13767 but it wasn't providing any value as best as I could tell.

This is a series of small improvements with the goal of:

1. Having the environment set up so all tests can be directly run from pytest, without needing to use nox
2. Having those tests be discovered by VS Code automatically and runnable from the GUI
3. Having VS Code extensions installed that matches the linting tools pip uses

This does that:

[ichard26]

If it works for me you (wow what a typo), LGTM.