Respect kit.vite.esbuild config when building with Vercel adapter

[mattjennings]

Is your feature request related to a problem? Please describe.
I have a dependency that needs to be excluded from the bundle, so I mark it as external in svelte.config.js like so:

const config = {
  preprocess: preprocess(),

  kit: {
    adapter: vercel(),
    target: '#svelte',
    vite: {
      esbuild: {
        exclude: ['external-dependency']
      }
    }
  }
}

This correctly gets excluded in the .svelte-kit build, but it gets included in the adapter build's output.

Describe the solution you'd like
Use the kit.vite.esbuild settings from svelte.config.js in the adapter build

Describe alternatives you've considered
I noticed the node adapter excludes pkg.dependencies - maybe we just need to do the same for Vercel adapters? Is there a reason for doing that in one adapter and not another?

How important is this feature to you?
Important, I'm unable to deploy until I can exclude this dependency.