Move server-side fetch responsibility to adapters instead of build step

[GrygrFlzr]

Alternative PR to #1034.

• Fixes adapter-cfw: server-side fetch fails due to dependency on Node's util.types #1019
• fetch is assumed to be provided by the environment, polyfilled by adapters if unavailable
• dev mode and prerendering polyfills using node-fetch as well
• Based off cross-fetch polyfill

Notes:

• Use of fetch inside server endpoints - do we want to re-export node-fetch or is this a userland problem?
  Endpoints now have de-facto access to global fetch
• Begin adapter still needs major updates so I haven't touched it

[Rich-Harris]

This is great. We also need global fetch in sveltekit-start (the tests were passing because they run build then start in the same process, but if you build an app then run start, it fails), so I moved the logic into a separate node-fetch-global.js module to DRY things out rather than repeating them a third time.

I almost wonder if we should expose that module to adapters somehow, rather than expecting them all to include that block correctly. It would also prevent things like version mismatches between dev and prod.

Re userland, I think it's solved by this PR? If fetch is available as a global to SvelteKit itself, then it's also available to endpoints. I've removed node-fetch from the realworld and HN examples accordingly.