Create a devcontainer feature to simplify install

devcontainers supports a [feature specification](https://containers.dev/implementors/features/), that simplifies adding something to a devcontainer base image (and removes the need for custom base images). More info on the background and philosophy can be found [here](https://code.visualstudio.com/blogs/2022/09/15/dev-container-features).

Presently there is a devcontainer for SWA CLI, but it might be better to ship as a feature.

https://github.com/devcontainers/feature-starter contains a starter template for creating it (and its tests).