Add Codespaces to the repository

[hummusonrails]

As discussed with @kianenigma, @PierreBesson and others, this pull request brings the ability to spin up a Codespace instance inside this repository. The PR has two components:

• devContainer.json: The setup instructions to initialize the instance with
• postCreateCommand.sh: A custom shell script to install the necessary dependencies for Substrate and the contracts node template on the Codespace instance that is invoked immediately after the instance is created

Once this is merged, we can then include links for folks to create their own Codespace with the contracts node template using their own GitHub account similar to the way Rustlings does do.

[athei]

CI fails because we need to sync against the latest substrate release I think.

[SkymanOne]

Yep. Please merge or rebase master for your branch

[hummusonrails]

Thanks both. I'll go ahead and do that early this coming week.

[hummusonrails]

@athei and @SkymanOne This is now ready for a review again. 🙏🏻