move config into svelte.config.js

[Rich-Harris]

I'm going to extract as much as possible out of #15844 and into their own PRs, so that it's less huge (and because it's useful independently of the await stuff)

starting with an easy one that I'll self-merge once green

[github-actions]

Playground

pnpm add https://pkg.pr.new/svelte@16265

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[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-svelte-16265-svelte.vercel.app/