[fix] polyfill fetch before running any app code

[geoffrich]

Fixes #3343

Before this change, referencing fetch at the top level of hooks.js would throw ReferenceError: fetch is not defined on platforms without native fetch. This is because we were polyfilling fetch after importing the compiled app, at which point it was too late. It looks like this was introduced with the adapter overhaul, where we stopped using esbuild's inject option.

To fix this, I moved the call to __fetch_polyfill inside its own module so that it would run before importing the app. I had to give it its own Rollup entry (in the adapters that use Rollup) so that it remained separate and import order was preserved.

I tested this locally with the node adapter and it fixed the linked issue. I also locally generated bundles with the Netlify and Vercel adapter and verified that __fetch_polyfill is called before any app code.

Please don't delete this checklist! Before submitting the PR, please make sure you do the following:

• It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC: https://github.com/sveltejs/rfcs
• This message body should clearly illustrate what problems it solves.
• Ideally, include a test that fails without this PR but passes with it.

Tests

• Run the tests with pnpm test and lint the project with pnpm lint and pnpm check

Changesets

• If your PR makes a change that should be noted in one or more packages' changelogs, generate a changeset by running pnpx changeset and following the prompts. All changesets should be patch until SvelteKit 1.0

[changeset-bot]

🦋 Changeset detected

Latest commit: 848f2aa

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 3 packages

Name	Type
@sveltejs/adapter-netlify	Patch
@sveltejs/adapter-node	Patch
@sveltejs/adapter-vercel	Patch

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[netlify]

✔️ Deploy Preview for kit-demo canceled.

🔨 Explore the source changes: 848f2aa

🔍 Inspect the deploy log: https://app.netlify.com/sites/kit-demo/deploys/61e6f6927bd42f00079473ff

[Rich-Harris]

ah whoops — thanks