Make grouped change files work with scoped publishing

@ling1726 pointed out in https://github.com/microsoft/beachball/pull/606#issuecomment-937760446 that grouped change files won't work well if there's a --scope specified when publishing (as we do now in fluentui) and the

There's not a great easy way around this. Some ideas:

• During a scoped publish, re-write the change file with only the packages that weren't included in the scope, and push that change along with the deletions of published change files.
• Promote scopes to a config-level concept (probably with names for easy reference when publishing) and make a separate grouped change file for each scope. However this doesn't solve if a package is included in multiple scopes (though we might be able to add checks/throw an error in that case).

Start with the linked pull request 606 comment and the grouped change-file behavior used by scoped publishing. Compare the two proposed approaches and define how packages inside and outside a --scope should be handled. Done means the chosen behavior is specified and its publishing impact is verified.