Always return something from render

[Rich-Harris]

This fixes #763 in two ways:

1. The render function in handle always returns a Promise<Response>, which means the developer no longer has to handle the case where nothing is returned because we tried to prerender a non-prerenderable page (in which case we now get a 204).
2. The dependencies object that used to accompany page responses now lives on the options that are passed in to app.render, bypassing the developer entirely. This eliminates a potential class of bugs arising from someone selectively returning properties of the response in handle.

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