Add experimental SSR support for the dev server

[FredKSchott]

Based off of work done in an internal project with @lukeed and in https://github.com/Rich-Harris/snowpack-svelte-ssr with @Rich-Harris

Changes

• Add an experimental API for the dev server (exposed as import {unstable__startServer} from 'snowpack')
• Add SSR support for the dev server via this new API.
• Add SSR support for plugins, and adds support to the svelte plugin specifically.

Testing

• Note: experimental/unstable means we do not yet document this in our docs, or commit to semver until it is no longer considered unstable
• Not exposed to the user, but tested manually in the projects mentioned above.

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[FredKSchott]

@Rich-Harris This file is honestly the best take on importing ESM from the dev server directly into Node.js that I've seen or can think of, at least until those ESM loader hooks are finalized: https://github.com/Rich-Harris/snowpack-svelte-ssr/blob/master/tasks/SnowpackLoader.js

It would be out of scope of all users of SSR to implement this themselves. How do you feel about us pulling this into Snowpack directly or shipping this as a separate utility helper package to help people integrate SSR with Snowpack?

[Rich-Harris]

I was secretly hoping you'd suggest something like that! The main reason I didn't bring it up yet was that the result of this approach isn't totally spec compliant (cycles, live bindings) and figured you might prefer something more robust if it's to be part of Snowpack. (Also, I realised that it caches too aggressively - modules aren't invalidated when their dependencies change. Need to fix that.)

But more broadly, yeah, I think it would be super useful to have this functionality as part of the API

[drwpow]

👍🏻