Versioning

As the spec continues to grow and evolve, some changes may be breaking. We should consider how to handle versioning so that the spec can continue to evolve without breaking users in implementing tools without their knowledge.

As one example, we've discussed various potentially breaking changes to Features ([example](https://github.com/devcontainers/spec/issues/109)). If our official Features get a breaking change, we can bump their major version, but we'll also want to consider how users of an implementing tool that surfaces this list will surface the appropriate version of the Features.

We could consider a similar versioning model to the package.json `engine` in [npm](https://docs.npmjs.com/cli/v9/configuring-npm/package-json#engines) or [VS Code](https://code.visualstudio.com/api/references/extension-manifest).