Prefer native fetch when available

[GrygrFlzr]

Really not happy with all the ts-ignore directives. Anyway.

• Fixes adapter-cfw: server-side fetch fails due to dependency on Node's util.types #1019
• Prefers native fetch when provided by the environment (Cloudflare Workers, Deno Deploy)
• Based off cross-fetch polyfill

Alternative solutions:

• Move server-side fetch responsibility to adapters instead of build step #1066 Move server-side fetch responsibility to adapters instead of build step

Unsolved problems:

• Use of fetch inside server endpoints - do we want to re-export node-fetch or is this a userland problem?
• This probably still includes unused node-fetch code in the built output for adapter-cloudflare-workers, which is a waste of space, bandwidth, and potentially cost.

[Rich-Harris]

I think this is a fine solution for now, though perhaps we could eliminate the overhead in the case where fetch is already provided by making it part of the adapter? i.e. each adapter is responsible for bringing a fetch implementation if one doesn't already exist.

So adapter-node might add an import in server.js...

import fetch from 'node-fetch';
globalThis.fetch = fetch;

...while cloudflare/deno/whatever would just skip that step entirely.

It does mean solving the problem n times instead of just once, but I think the solution is cleaner, and it means that users don't need to worry about it.

[benmccann]

FYI, this is why we had to use beta version of node-fetch: #467

[GrygrFlzr]

Created an alternative PR #1066 following Rich's suggestion to move the node-fetch polyfilling into adapters instead.

[Rich-Harris]

closing this in favour of #1066