chore: add workflow for changelog label

[leogermani]

All Submissions:

• Have you followed the Newspack Contributing guideline?
• Does your code follow the WordPress' coding standards and VIP Go coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

Adds a Workflow that will add a label and a comment to a PR after it's merged to trunk.

Context: pemhSX-De-p2

How to test the changes in this Pull Request:

Well, the only way to test it is merging :) Read the code

You can see how it looks like here

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully ran tests with your changes locally?