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).

Start by reviewing the existing SWA CLI devcontainer and the devcontainers/feature-starter template, including its tests. Determine how the current devcontainer installs the CLI and use the template's test flow to verify the feature. Done means the SWA CLI can be added to a devcontainer base image through the new feature.

索引モデルが issue の本文から書いたものです。