[css-fonts-4] Add syntax for <font-src-list>

[pyoor]

It would be useful to include to explicit syntax for <font-src-list> rather than just describing it in the prose:
<font-src-list> = <font-src>#

https://github.com/w3c/csswg-drafts/blob/7ebecab15e4ecc288a70c9ba410215256cce0865/css-fonts-4/Overview.bs#L2499C8-L2499C8

[cdoublev]

invalid is not a valid <font-src> but invalid, url(font.woff2) is a valid <font-src-list>, therefore [<font-src> | <any-component-excluding-comma>]# would be more accurate than <font-src>#.

However, <media-query-list>, <forgiving-selector-list>, <block-contents> and its equivalent but more restrictive productions, are not expanded.