feat: add config option to prevent import into client code

[Dudek-AMS]

Closes #12477

Added ~~serverProtectedPaths~~ serverOnlyPaths to add additional rules to prevent import into client code

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Latest commit: 82f1df281a2fa2d74f9ddf69f51923efcb70e51f

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[Dudek-AMS]

Ive dropped the changeset so far to be added by maintainers.

[Dudek-AMS]

After reconsidering the naming, I would suggest naming the attribute serverOnlyPaths

[dominikg]

3 different ways to define user-supplied rules seems a bit much, esp. if there is ()=>boolean already, users can just use regex or string compare themselves. But before continuing on this PR please see my comment in #12477

[Dudek-AMS]

3 different ways to define user-supplied rules seems a bit much, esp. if there is ()=>boolean already, users can just use regex or string compare themselves. But before continuing on this PR please see my comment in #12477

the idea was to make the function method for more complicated rules, while string is prefered. ofc function only does work out

[Rich-Harris]

closing per https://github.com/sveltejs/kit/issues/12477#issuecomment-3188409462