fix(mc): handle standard merge fields when fixing duplicates

[dkoo]

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:

Allows our script to consolidate and fix duplicate merge fields to also handle standard Mailchimp merge fields (First/Last Name, Phone, Address) in addition to Newspack-specific fields.

How to test the changes in this Pull Request:

1. In your connected Mailchimp audience, create duplicate merge fields for First Name, Last Name, Phone, and Address. These fields usually exist for every Mailchimp audience and can also be a part of our contact metadata payload, which means they could be duplicated by our sync methods.
2. Fill out these fields for some existing contacts in your audience.
3. Run wp newspack mailchimp merge-fields fix-duplicates --dry-run and confirm that the duplicate fields are reported.
4. Run wp newspack mailchimp merge-fields fix-duplicates and confirm that the duplicate fields are deleted in Mailchimp, and that the info you filled out in step 2 get consolidated into the original instances of those fields.

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?