feat: Add support for bzlmod

[mortenmj]

This PR adds a MODULE.bazel file which allows other repositories to import this one as a bzlmod module.
I don't know how Google handles publishing to the Bazel Central Registry, so I haven't made any attempt at that.

I've added a module extension to grab rules_jsonnet, as it isn't available yet as a module, and to create the python header repository.

All tests are successful when run with --enable_bzlmod, and running jsonnet/jsonnetfmt from //cmd succeeds.

I've also made a similar PR for go-jsonnet, which depends on this bzlmod module. That PR can be found here: google/go-jsonnet#698

[mortenmj]

@sparkprime could you review this PR?

[mortenmj]

@sparkprime looks like adding this to the Bazel Central Registry requires that the release process explicitly creates a source archive. The generated ones are not guaranteed to be stable, and cause a test failure when submitting to BCR.

https://buildkite.com/bazel/bcr-presubmit/builds/1361#018857db-81f5-4ff0-b07c-05f5587d41a3

BcrValidationResult.FAILED: jsonnet@0.20.0 is using an unstable source url: https://github.com/google/jsonnet/archive/refs/tags/v0.20.0.zip.
  | You should use a release archive URL in the format of https://github.com/<ORGANIZATION>/<REPO>/releases/download/<version>/<name>.tar.gz to ensure the archive checksum stability.
  | See https://blog.bazel.build/2023/02/15/github-archive-checksum.html for more context.

Is that something you'd be willing to take a PR to do?
The solutions I've seen to this are either a GitHub Action that does the thing, or various ways of doing it manually.

Edit: The request to add Jsonnet to BCR is here: bazelbuild/bazel-central-registry#667