rename __fetch_polyfill to installFetch

[Rich-Harris]

Closes #4098

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🦋 Changeset detected

Latest commit: 948d10f

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

This PR includes changesets to release 4 packages

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

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