feat: add font-loading class

[adekbadek]

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 newspack--font-loading class to the body element until custom fonts are loaded. This will a enable font-loading-related performance improvement.

Let's say a site is using a thick font for headings. In order to prevent layout shift on font load, the fallback font can be styled to match the thickness (e.g. by setting letter-spacing to something wide). On font load, the body class will be removed, and the custom font displayed as expected.

How to test the changes in this Pull Request:

1. In Customizer -> Typography, set "Font Provider Import Code or URL" to e.g. //fonts.googleapis.com/css2?family=Anton&family=Inter:wght@400;500;700&display=swap, then set "Header Font" to Anton
2. In Customizer -> Custom CSS, add e.g. .newspack--font-loading h3 { letter-spacing: -1px; }
3. Load the page, observe initially the body element has the newspack--font-loading class, which is then removed. The h3 elements should match the Anton font on load. Enable network throttling for better observation.

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?