[Styles] Webfonts enqueued using styles JSON files are not loaded

[kjellr]

Webfonts declared in custom styles json files are not enqueued. No error is thrown, but the fonts are not loaded in.

---

To reproduce:

1. Add a style variation to emptytheme.
2. Add font file(s)
3. Declare the font using fontFace under settings.typography.fontFamilies.
4. Open the Site Editor.
5. Select the variation, notice that the correct font is not loaded.

Example PR here: WordPress/theme-experiments#304

In the PR example, the same code works properly when pasted into theme.json instead of styles/variation.json.

---

Screenshots:

The correct font for the variation should be DM Sans. Instead, the default sans-serif fallback is being used, and no network call to the DM Sans font files is being made.

[jasmussen]

@youknowriad @aristath @jorgefilipecosta have you worked with fonts, or have an idea what might be up here?

[youknowriad]

I guess we only load the fonts that are on the main theme.json and not the variations. It's going to be important to solve this in a way that lazy-loads these fonts. (In case variations will come from a directory later)

[aristath]

huh... tbh I wasn't even aware of the existence of variation.json files! 😬
Where do we parse these? I can't seem to find any reference to variation.json in Gutenberg's code 👀

[kjellr]

👋 @aristath style variations were implemented over in #35619

[aristath]

Thank you for the additional context @kjellr 👍
I added a PR for this in #39886