build: add commit message scope for multiple components

[crisbeto]

As discussed, adds a new multiple scope for commits that cover more than one area.

[josephperrott]

I will leave it to you all since this repository does not rely on the commit messages from a PR to create commit message. My personal perspective is that if a change needs to affect multiple areas, it should be split into multiple specific commits. Since these commits are used to generate our release notes, it is best for us to have specific targeted scopes for our commits so that changes appear as best as possible in our release notes.

[jelbourn]

LGTM

I think it makes sense to have this when a refactoring would be too cumbersome to have per-component commits (e.g., making all @Output members readonly)

[devversion]

Just FYI: We updated the dev-infra code to accept refactor and test commits without scope. angular/angular@f98f379.

I can still see multiple being useful for other commit types.

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