Online "playground"

I'd like an online playground app where people can create c42 policies, run them against arbitrary files, etc.

Would also like it to have a bunch of sample content that can be loaded, all to give people a good sense of the tool, and the ability to play around with various aspects of it.

The issue does not name any files, tests, or entry points. First define the playground's scope, execution model, sample content, and success criteria with the maintainers; no implementation starting point or definition of done is provided.

Written by the indexing model from the issue text.