feat: warn on external goto navigation

[dummdidumm]

in preparation for SvelteKit 2
related to #8775 / #11207

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[dummdidumm]

Mhm ... I just did this PR, but now I'm wondering how much use the warning really is - you'll probably never see it because you navigate away from the page. Probably doesn't hurt though, and having the new option in SvelteKit 1 already is good.

[dummdidumm]

Now that we decided against making this an option, there's not much use in this PR anymore, as rarely anyone will see the warning, therefore closing.