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

[eitsupi]

Description

I am excited to see the Repo link feature trying to support Dev Container / GitHub Codespaces. c7803ea

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

[mcanouil]

Closing in favour of:

• [code-links] Support devcontainer.json links to sub folder #6717