feat: expose isFileServingAllowed as public utility

[antfu]

Description

This would be beneficial for meta-frameworks to check if some file/url is safe to serve or short-circuit logics on those files. Using the same configuration from Vite.

Namely, for us, we need it on Nuxt to filter out accessing arbitrary files in the FS with our custom middleware. (Potentially the Vitest browser environment a well)

Additional context

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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