fix(popups): improve formatting of human-readable numbers

[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:

Small fix to improve the display formatting of human-readable "Seen", "Form Submissions", and "Link Clicks" numbers in Campaigns Analytics according to the following rules:

• If the value is less than 10k, shows the absolute value formatted according to the local language via toLocaleString().
• If the value is greater than 10k but less than 1M, shows a human-readable figure rounded to the nearest integer. e.g. 11,236 becomes 11K and 157,963 becomes 158K
• If the value is greater than 1M, shows a human-readable figure rounded to the hundredths. e.g. 1,353,288 becomes 1.35M and 826,677,234 becomes 826.68M.

Closes Automattic/newspack-popups#835.

How to test the changes in this Pull Request:

1. If your test site isn't connected to Google Analytics via Site Kit, you can force the Campaigns Analytics view to display by adding return []; at this line.
2. Mock various numbers by altering this line like so:

// const { seen, form_submissions, link_clicks } = keyMetrics;

// Mock numbers for testing.
const seen = 10000;
const form_submissions = 1023;
const link_clicks = 12456;

3. Test numbers in the singles, hundreds, thousands, tens of thousands, hundreds of thousands, and millions and confirm that the numbers displayed are human-readable and make logical sense according to the rules above.

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?

[miguelpeixe]

Nice! 💯

[matticbot]

🎉 This PR is included in version 1.82.0-alpha.1 🎉

The release is available on GitHub release

Your semantic-release bot 📦🚀

[matticbot]

🎉 This PR is included in version 1.82.0 🎉

The release is available on GitHub release

Your semantic-release bot 📦🚀