next/font: tolerate missing unused fields in capsize map (#50708)
Some font entries in capsize do not include fields like `cap_height` and `x_height`. Since these are unused by next/font, remove these from the serde structure entirely.
Additionally, this makes it so that failing to read or deserialize the capsize map results in an `Error` rather than a `FontFallbackError`, which causes a build failure rather than silently omitting a fallback font. `next/font` should always include this map, and it obscured the real issue here. However, a missing entry in this map _should_ result in omitting a fallback.