Should the namespace of `customizations` be more standardized?

Currently, customizations are namespaced by arbitrary product names. The only two I've seen in wide-use are `vscode` and `codespaces`, which makes sense as those are the only two widely used implementations of devcontainers.

As the spec is picking up steam, it may be worth considering changing these keys to use [reverse domain name notation](https://en.wikipedia.org/wiki/Reverse_domain_name_notation). Implementing tools would continue to support the original names for the foreseeable future, but could map the legacy name to the proper name and potentially print a warning at build time at some point in the future if the customization namespace does not look like a reverse domain, with extra flavor for well-known names.

Thoughts? If there's interest I could write it up as an actual proposal.

Start with the customizations namespace in the devcontainer specification and compare the existing vscode and codespaces keys. Decide whether reverse domain name notation should replace or supplement legacy names, including migration and warning behavior. Done means the specification change and compatibility policy are agreed and documented.

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