feat(vite): support custom configFile path for sveltekit plugin

[axel7083]

Fixes #13748

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Latest commit: 3c68754

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@sveltejs/kit	Minor

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[benmccann]

The motivation here isn't clear, but this looks like something to support monorepos? That's come up several times in the past (#13027, #12420, #12410) and folks thought it would be best for it to be addressed via the monorepo tooling: #12499 (comment)

[benmccann]

Closing this given the lack of response and feedback from other maintainers on similar PRs in the past