Add a `playground` command to `IntegrationTestModule`

[lefou]

The idea is to fully explode the example project into a given (or default Task.dest) directory, so that the user or developer can just cd into that project and inspect it. This helps proofreading, testing, experimenting. We could even automatically init a local git repo, so any changes can be easily inspected.

[lefou]

One maintenance use case is to (bulk) detect outdated dependencies in example projects by using some command to find dependency updates in every example project.

If we allow to run some free command in a playground dir, we could run:

> mill example.some.project.playground --run mill mill.scalalib.mill.scalalib.Dependency/showUpdates

[lefou]

Also spawning a shell in that project would be great.