Add missing value to CSS font-display descriptor in theme.json

[pagelab]

What?

The “optional” value for the CSS font-display descriptor is missing from theme.json (see below). This PR simply adds it.

Why?

The value is part of the CSS fonts spec, and it's recommended for body text, where a custom font is not absolutely required to load on the first visit and can be safely seen only on subsequent page loads.

How?

The theme.json schema was manually updated with the missing value.

Testing Instructions

1. Open any theme.json file, from any block theme, on VS Code.
2. Go to the fontFamilies section and add a fontDisplay definition.
3. Add a new value and see that “optional” appears.
4. Check the front-end code to see if the value was correctly applied to the font-face property.

[aristath]

Thank you for this PR!
I backported it to the WIP refactor on #43492 as well, so we don't miss it when the time comes for that PR 👍