feat: Add CSP support for hydratable

[elliott-with-the-longest-name-on-github]

This also gives us an avenue for supporting CSS stuff in the future.

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Playground

pnpm add https://pkg.pr.new/svelte@17338

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Preview: https://svelte-dev-git-preview-svelte-17338-svelte.vercel.app/

[elliott-with-the-longest-name-on-github]

as of now, this will only ever have one hash in it -- however, with streaming, it would be more...

Thinking ahead, with streaming, this should probably error if you try to access it prior to fully-reading tail, because it needs to be fully-populated before you send the response.

[elliott-with-the-longest-name-on-github]

I dedented and changed the formatting of this stuff because it was confusing in the browser... I could be convinced to sacrifice browser readability for code readability but I don't think the readability here is that bad, and it also technically reduces the byte count