Define a standard rule to concat all sources

Someone reported this issue inside Google:

> It looks like there's call for a shared common rule that simply cat's all sources.
>
> It's even the example for genrule in the build encyclopedia:
> https://docs.bazel.build/versions/master/be/general.html#genrule_examples

Start with the linked Bazel build encyclopedia genrule example and inspect bazel-skylib's existing rule conventions. The issue names no files or tests; clarify the shared rule's interface and what counts as completion before implementation, then add appropriate validation for the agreed behavior.

Written by the indexing model from the issue text.