chore: cleanup promise handling

[Rich-Harris]

#12284, without all the breaking changes and stuff

there's a cryptic error, i have no idea what this means:

/Users/rich/Development/SVELTE/KIT/kit/packages/adapter-vercel/index.js
  0:0  error  Parsing error: "parserOptions.programs" has been provided for @typescript-eslint/parser.
The file was not found in any of the provided program instance(s): packages/adapter-vercel/index.js

[changeset-bot]

🦋 Changeset detected

Latest commit: 136e7d4

The changes in this PR will be included in the next version bump.

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Name	Type
@sveltejs/enhanced-img	Minor

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[benmccann]

there's a cryptic error

That will be fixed by #12280, which should be merged before this PR. I was hoping you or Simon could give it a look to confirm my understanding of the Vercel APIs as I couldn't quite glean from the docs if my change is correct

[benmccann]

I added a TODO for a config cleanup I wanted to do and can't figure out

I would still like to understand some of the other rules better, but it seems worth merging this with the less controversial stuff and handling the other ones separately, which should make things easier to discuss