Bun runtime on Vercel Functions

Introduces Bun as a new runtime option for Vercel Functions, available in Public Beta. Details performance improvements (28% latency reduction in CPU-bound Next.js workloads) attributed to Bun's architecture. Explains how to configure the Bun runtime via `vercel.json` and highlights its compatibility with Next.js, Express, Hono, and Nitro. Discusses performance characteristics and tradeoffs between Bun and Node.js, emphasizing testing dependencies before production migration. Includes benchmark results comparing full TTFB for server-side rendering workloads.