Turbopack next/font/google: Use capsize's xWidthAvg to compute fallbacks (#50878)
Instead of using azAvgWidth for next/font/google, use xWidthAvg as the webpack implementation does. For now, next/font/local in both the webpack and Turbopack implementations continue to use azAvgWidth. We should align these in the future.
Test Plan: We now pass 26 next/font tests in Turbopack, up from 22.