Do not generate changelog entries for private packages

[NullVoxPopuli]

Alternate to: embroider-build/release-plan#60
(and more robust of an alternative than what's going on in that release-plan PR)

Resolves: embroider-build/create-release-plan-setup#91

Try this out via:

"pnpm": {
  "overrides": {
    "@ef4/lerna-changelog": "github:embroider-build/github-changelog#dist/ignore-private-packgaes",
  }
}

or

"pnpm": {
  "overrides": {
    "@ef4/lerna-changelog": "github:embroider-build/github-changelog#3ad92f5",
  }
}

Example: NullVoxPopuli/limber#1691

Outputted: NullVoxPopuli/limber#1673

[mansona]

So this feels closer to what we actually want to do 🎉 It's a long-standing issue in the pre-forked lerna changelog lerna/lerna-changelog#128 so I would really like it if our github-changelog could actually fix this for people and encourage them to switch 💪

This means that we should carefully read the issue and make sure that we cater for what people were asking for too 👍 and we need tests before we can merge this since it's a serious change 🙈

I'm requesting changes so we know we need tests before merging