Exclude prerendered pages from serverless functions/workers

[Rich-Harris]

Is your feature request related to a problem? Please describe.
There's no sense in shipping the code for routes that got prerendered; they will never be called on during SSR. We can't guarantee that of routes with dynamic parameters (we might have prerendered /blog/one and /blog/two but we might need /blog/[slug].svelte in order to render /blog/three in future), but we can for other cases. It's a small win but worth having.

Describe the solution you'd like
I have some half-baked ideas for what the API could look like but I should probably get back to writing my Svelte Summit talk before I explore all the ways in which those ideas are bad

How important is this feature to you?
It's a nice-to-have. The current situation is wasteful, but not egregiously so