Create devcontainer.json

[rafaelstz]

WHY are these changes introduced?

It introduces dev container to the demo store to make it easier to use.

Checklist

• I've read the Contributing Guidelines
• I've considered possible cross-platform impacts (Mac, Linux, Windows)
• I've added a changeset if this PR contains user-facing or noteworthy changes
• I've added tests to cover my changes
• I've added or updated the documentation

[blittle]

Hi @rafaelstz, thank you for the PR. Can you explain to us the benefits of having this in the repo itself, instead of someone using devcontainers just adding their own devcontainer config?

[rafaelstz]

Hi @blittle, this would take less time to start the project using VSCode or Github CodeSpaces. Since we have the minimum requirements to run Hydrogen, having the standard setup created would avoid issues with the environment to start testing it.
It wouldn't avoid customizations too, as same as .gitignore file.