[feat] allow using Vite's strict.port option

[benmccann]

Closes #2467

This delegates port checking to Vite and allows you to set strict.port: false. I set the strict.port default to true, which is different than Vite's default value for backwards compatibility

The server startup message is printed twice, but that will be fixed when we upgrade to Vite 2.6. That upgrade will remove their startup message and leave only ours

This doesn't touch preview mode for now. It may be possible to use the Vite server in preview mode as well, but that would be blocked by an upgrade to Vite 2.6

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@sveltejs/kit	Patch

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[Conduitry]

Is there any cleanup we can do now - or would stuff like removing port-authority as a dependency happen after we also (hypothetically) switch to using Vite's server for preview as well?

[benmccann]

Yeah, I don't think we could remove port-authority yet since it's still used in preview mode. Not sure if you had any other ideas for cleanup, but I don't think there's a whole lot more we could do at the moment