Create playbook from antora-playbook.yml + a patch file

[osfameron]

Why

We maintain a bunch of playbooks locally, or for other environments, which eventually get out of sync with our main branch.
This means e.g. we forget to add new branches or extensions to Staging, or forget to update our ui-bundle locally etc.

When we have a local playbook that works, it's still a hassle to manually create the remote playbook for previewing.

Use Cases

1. Turn a local playbook into a remote playbook (for previewing)

$ scripts/remotify my-playbook.yml
...
Composing antora-playbook.yml with my-playbook.remote.patch.yml
Written my-playbook.remote.yml

2. Create a local file:

scripts/patch-playbook.mjs --patch patch.yml > local-playbook.yml
npx antora local-playbook.yml

See example patch.yml in this PR.
Operators like $filter, $prune, $local will cut down to certain repos, limit to a manageable number of branches, and turn the remote refs into local ones if you have them checked out.

3. Create a staging playbook

scripts/patch-/playbook.mjs --patch patch-staging.yml > antora-playbook-staging.yml

This example again uses $prune to limit to the most recent X branches.
We can also $override individual repos, to add particular branches/start_paths etc. (e.g. for the devex private preview)