make static assets immutable

[Rich-Harris]

first part of #3194

• adapter-cloudflare
• adapter-cloudflare-workers
• adapter-netlify
• adapter-node
• adapter-vercel

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🦋 Changeset detected

Latest commit: 3165f99

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

This PR includes changesets to release 5 packages

Name	Type
@sveltejs/kit	Patch
@sveltejs/adapter-cloudflare	Patch
@sveltejs/adapter-cloudflare-workers	Patch
@sveltejs/adapter-netlify	Patch
@sveltejs/adapter-vercel	Patch

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[Rich-Harris]

cases this doesn't (yet) address:

• paths.assets is set, meaning assets are hosted externally. in these cases, manifest.assets should be empty, and adapters probably shouldn't attempt to deploy or serve static files or write custom headers for them
• paths.base is set. adapters are probably doing several wrong things here

i think it's probably better to figure those out separately