Add VS Code dev container configuration.

[smarnach]

This PR adds a dev container configuration for VS Code. Features:

• All dependencies are installed in the dev container, so IntelliSense fully works, and type checking will also work once we add type annotations.
• Allows running the tests using VS Code's Python test browser, which makes running and debugging tests really convenient. (This is the main reason to use a container instead of a virtualenv in the first place.)
• Sets up Black as the default Python formatter. We can consider adding a setting to automatically format Python code on save.

I'm not really an expert on VS Code, so it's possible there are better ways of setting this up. This configuration works well for me, though, and it might be useful for others as well.

[biancadanforth]

Thanks Sven for these improvements! Like you, I am not a VS Code expert, but I agree this is a nice improvement for devs using VS Code.

I do have one request to add a small docs section for VS Code on using the Dev Containers extension. Perhaps we can add a .vscode/extensions.json file with a recommendations field pointing to the Dev Containers extension as well (referencing something similar I remember as a dev for FxA)?

[smarnach]

@biancadanforth I added some documentation and the recommendation for the dev container extension. Please take another look.

[biancadanforth]

LGTM! Made a couple of small suggestions on the docs. Thank you.

[smarnach]

I found a bug in the configuration. Since the devcontainer service inherits from the test service, it also inherits the image name tecken:build from that service. As a result, building the dev container overrides the tag for the Tecken image, and if you stop and restart the whole stack, the dev container is used for any service inheriting from base. I fixed this by using a different image name for the dev conatiner.