build: group changelog entries by packages

[devversion]

Resolves COMP-225

[jelbourn]

What do the commit messages need to look like with this?

[devversion]

@jelbourn I've updated the CONTRIBUTING.md file as part of this PR. That should make it clear what commit format we expect. This matches the format we already used for cdk-experimental and material-experimental changes.

If we decide to go with this, I'd propose that we also lint the commit message similarly to how it is done in angular/angular.

[josephperrott]

I think that regardless of what changes we make in CHANGELOG we should begin linting commit messages

[jelbourn]

LGTM

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