DX: Document or improve variable font behaviour

[JakeIsMeh]

"Demo":
https://github.com/JakeIsMeh/fonts-repro

Currently, there are no conventions on how to name variable versions of fonts, which can lead to many issues when users try to specifically select variable versions of fonts.

Fontsource has it's users specify [font name] Variable to use variable fonts, while Inter currently has users specify InterVariable (v4, from the website) or Inter var (v3).

While I don't think that it is unreasonable for the OOTB behavior to expect plain names that are searchable across all providers, I think that this should either be called out in the docs or the parser becomes smart enough to process this (or warn when a font cannot be resolved).

Additionally, I believe that this, #172, and a perfect storm of a locally installed version of Inter contributed to nuxt/nuxt.com#1589, showing that it is already a footgun in real-world scenarios.