select adapter immediately on startup

[Rich-Harris]

While working on #4289, I found a use case for adapter-auto determining the adapter early, rather than waiting until adapt is called. I think this is very likely to come up in other ways, so I figured we may as well come up with a solution.

In this PR, the checking is done as soon as the module executes.

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🦋 Changeset detected

Latest commit: ba018d0

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

This PR includes changesets to release 2 packages

Name	Type
@sveltejs/kit	Patch
@sveltejs/adapter-auto	Patch

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