Add devcontainer setup with a test CKAN

[florianm]

As a contributor, it's an entry barrier to setup a test CKAN.

To facilitate contributions, ckanr could:

• Add a devcontainer setup
• Add the existing docker-compose file to the devcontainer to provide a running test CKAN instance
• Add VSCode extensions to assist with R package development
• Add VSCode settings to improve developer experience and to nudge developers towards best practice
• Add a pre-commit config with (commented out) hooks to format and lint the codebase

Example with explanations: florianm/synthpop#2

As an outcome, a contributor could

• Fork ckanr and launch a Codespace
• Have a working, disposable CKAN instance available in the codespace (write without regret)
• Have pre-commit enforce code formatting (never worry about formatting again)

The developer experience in VS Code is obviously different from the widely adopted RStudio Desktop.