We achieve this by making properties that accept a custom-ident value
skip the "someone else's vendor prefix" check for values that start with
a `-` character.
This fixes an issue where e.g `font-family: Arial, -apple-system` would
be rejected by the parser completely. We now treat `-apple-system` like
an identifier in such cases.
Also add `valid-types` metadata for the `font-family` property so this
actually works. :^)
