breaking: replace builder.generateManifest with builder.generateServer and builder.getManifest

[ottomated]

alternative to #16827

Removes SSRManifest as a public api. Previously, adapters were using it directly; now they have a more limited api that mirrors $app/manifest. This ends up simplifying the code that adapters have to generate, because they only have to worry about a single server module vs server + manifest. It also removes the need for manifest._.foo in the internal code, which is nice.

Technically, SSRManifest is still used by some adapters (search server.manifest to find the uses). I don't know if we want to go all the way here and replace those with, i.e. rolldown defines?

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Install the latest version of @sveltejs/kit from da22759:

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🦋 Changeset detected

Latest commit: da22759

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

This PR includes changesets to release 5 packages

Name	Type
@sveltejs/adapter-cloudflare	Major
@sveltejs/adapter-netlify	Major
@sveltejs/adapter-node	Major
@sveltejs/adapter-vercel	Major
@sveltejs/kit	Major

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Preview: https://svelte-dev-git-preview-kit-16843-svelte.vercel.app/

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Additional Suggestion:

Netlify edge.js entry still uses the old new Server(manifest) API, so Server is undefined and the MANIFEST import is unresolved, breaking the edge function build.