[code-links] Support devcontainer.json links to sub folder

(Note - we currently detect devcontainers in subfolders, but do not form links to them, instead forming a global devcontainer link and letting GitHub sort it out)

Discussed in https://github.com/quarto-dev/quarto-cli/discussions/6683

^{Originally posted by eitsupi September 2, 2023}

Description

I am excited to see the Repo link feature trying to support Dev Container / GitHub Codespaces. c7803eab16a217cc3fd26a2039db7c090cedd8f0

By the way, note that multiple Dev Container definitions can be included in a single repository.
The Spec says:

• .devcontainer/devcontainer.json
• .devcontainer.json
• .devcontainer/<folder>/devcontainer.json (where is a sub-folder, one level deep)

https://containers.dev/implementors/spec/#devcontainerjson

In fact, we can create demos that include links to multiple Codespaces in a single repository, such as:
https://github.com/eitsupi/devcontainer-try-r

Start by tracing the code-links entry point that detects devcontainers and currently forms a global link. Compare link generation with the devcontainer.json specification and confirm the expected behavior for one-level subfolders. Done means each supported subfolder definition produces its own repository link, including repositories with multiple Codespaces definitions.