ssr/router/hydrate options (SPA mode! 0kb JS pages!)

[Rich-Harris]

See #231 (and also #570). I'm practicing readme-driven development by writing the docs first and worrying about the implementation later, so that folks can weigh in on the design straight away.

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[Rich-Harris]

Ok, this is implemented. Kind of fun to add export const hydrate = false, router = false to a page and see it render without loading any JS whatsoever. Maybe one day we'll be able to determine that automatically through static analysis, but in the meantime 0kb JS is a nice selling point.

Just need to figure out if we want to make any tweaks to the API. (@benmccann I like the SSR-for-googlebot idea but I think we can work that out in a separate PR)

[frederikhors]

@Rich-Harris I do not know how to thank you! ❤️

[meigo]

Is it certain it works?
On page setting

<script context="module">
  export const hydrate = false;
  export const router = false;
</script>

and still getting js in prerendered html

<link rel="modulepreload" href="/./_app/start-7d9b1f5f.js">
<link rel="modulepreload" href="/./_app/chunks/vendor-b2527fc7.js">
<link rel="modulepreload" href="/./_app/pages\index.svelte-1c557d65.js">
...

Both with node and static adapters.

[frederikhors]

@meigo we are all waiting for it: #754!

[meigo]

Oh, I see, thanks!