add devcontainers configuration

[johannesjh]

suggestion to add a devcontainers config to this repo.

Motivation: A devcontainers configuration would allow contributors to develop and build fava without having to mess with package installations on the local machine.

Specific example configuration: The following devcontainers configuration worked fine when I gave it a try.

// For format details, see https://aka.ms/devcontainer.json. For config options, see the
// README at: https://github.com/devcontainers/templates/tree/main/src/debian
{
	"name": "Debian",
	"image": "mcr.microsoft.com/devcontainers/base:bullseye",

	// Features to add to the dev container. More info: https://containers.dev/features.
	"features": {
		"ghcr.io/devcontainers/features/python:1": {},
		"ghcr.io/devcontainers/features/node:1": {}
	},

	// Install additional packages as needed
	"postCreateCommand": "pip3 install --user build twine"

	// Use 'forwardPorts' to make a list of ports inside the container available locally.
	// "forwardPorts": [],

	// Configure tool-specific properties.
	// "customizations": {},

	// Uncomment to connect as root instead. More info: https://aka.ms/dev-containers-non-root.
	// "remoteUser": "root"

}