Make .svelte-kit configurable as outDir

[Rich-Harris]

First part of #4052. It's not a complete fix, since if outDir is outside the project directory (which could be true in a monorepo), the paths.js module will be imported twice, which will break stuff if you have a paths config. This is something of an edge case so I didn't want to hold up the PR for it, but we can keep #4052 open in the meantime.

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