Remove external from adapter-vercel?

[Rich-Harris]

adapter-vercel has an external option that I'm pretty sure is useless. It's used when creating edge functions, which are bundled with esbuild, but I'm 99% sure that edge functions won't work if you have external dependencies — everything needs to be bundled together.

As far as I can tell, the option is vestigial — we needed it at one time because we were using esbuild for serverless functions, and sometimes you'd need to exclude certain dependencies like sharp. But we no longer use esbuild in that context, so I don't believe there's any reason to keep it.

Am I missing anything?