feat: add warning for possible route filename typos

[eltigerchino]

closes #10528

Outputs a terminal warning if a route filename is missing a + or if the default hooks filename (src/hooks.server) mistakenly has a + prefix.

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Latest commit: ad00777

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@sveltejs/kit	Minor

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[eltigerchino]

Sorry for the delay. All the suggested changes are now included.

[dummdidumm]

Thank you!