build: add commit message scope for migration changes

[devversion]

This is a proposal commit that adds a separate scope for migration changes.

The motivation is that migrations aren't necessarily always affecting @angular/core
(e.g. the move-document migration is affecting platform-browser). Migrations are just
stored in the @angular/core package in favor of a canonical location when someone
runs ng update. Additionally, it rather seems confusing that migration changes are listed
under core in the changelog. Long commit message titles are needed to make clear that
commits do not affect @angular/core runtime logic, but just the ng update migrations.

[gkalpak]

OOC, why not use the proper scope (instead of core) in the commit messages (even if the changes are in packages/core/)?

[devversion]

That's a good point. I think it would be still ambiguous if we would use e.g. platform-browser if the migration affects the platform-browser package as the actual changes are made in packages/core/.

Additionally it would still require longer commit messages, and in the changelog, migration changes would not be grouped. I guess it would be good to have migrations show up separately in the changelog as they don't affect core runtime issues. Similarly to the ngcc scope that could be just compiler-cli too.

[devversion]

@IgorMinar Would you be able to provide some feedback on this? It's not urgent though 😄

[IgorMinar]

SGTM but we should announce this change and in order to enforce it we will need reviewers to look for the current scope to be applied. Changing the pullapprove settings to create a new group for migrations would make that easier.

[devversion]

@IgorMinar Thanks for reviewing! Yeah, we added a separate code owner group for migrations already (see here).

Definitely agreed that we should announce this. I'm wondering if we could do better at enforcing these commit scopes. That might be a good thing to look into from the dev-infra side.

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