Instead of getting the name from the optional `'name'` field, the font-family name now comes from the required `'fontFamily'` field.
This change fixes a back-compat (BC) break in how the font-family name is pulled from the incoming font data in the `WP_Font_Face_Resolver`.
Why?
WP Core does not require the `'name'` field in theme.json. For themes that do not declare it, that set of font variations is ignored, thus causing a BC break from how the stopgap code worked (see [53282]).
However, `WP_Theme_JSON` schema does require the `fontFamily` field in each of the `typography.fontFamilies`.
== Other details:
Includes a parser to extract the first entry when a `fontFamily` field has a comma-separated list of font-families, e.g. `Inter, sans-serif`.
References:
* Merge from Gutenberg's PR https://github.com/WordPress/gutenberg/pull/54615.
Follow-up to [56500], [53282].
Props ironprogrammer, hellofromTonya, mmaattiiaass, pbking.
Fixes#59165.
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