[fix] discover new routes in dev server

[jasonlyu123]

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Fixes #2046. The manifest data passed into the server handler it's the initial manifest. And the manifest won't be mutated by reference but resigned. This PR changes to pass a get function to the handler so it'll get the up-to-date manifest.

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🦋 Changeset detected

Latest commit: e357735

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

This PR includes changesets to release 1 package

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@sveltejs/kit	Patch

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[ignatiusmb]

Looks like the tests are finally passing, this would be a really nice improvement for the dev server!

[benmccann]

awesome!