Announcing the Build Output API

Introduced the Build Output API, a file-system-based specification allowing any framework to build for Vercel and leverage Vercel's infrastructure primitives (Edge Functions, ISR, Image Optimization, etc.). Added `vercel build` to the CLI for local build debugging and inspection, and `vercel deploy --prebuilt` to decouple the build step from deployment, enabling deployment of pre-built outputs without sharing source code. This empowers framework authors to integrate Vercel's infrastructure primitives more easily.