feat: whitelist external remote functions

[ottomated]

(retargeting #14028)

fixes #13979

Adds a new config option, remoteFunctions.allowedPaths, which allows remote functions to be loaded outside of src.

I like this approach because:

• It's pretty simple to implement
• It follows the pattern of stuff like vite & pnpm, requiring explicit authorization
• Because users whitelist whole folders at once, they can easily allow a whole npm package
• It's way more performant than walking the entire node_modules directory recursively to detect remote functions there
• The warning when a user attempts to import remote functions that aren't whitelisted is intuitive and easy to fix:

Also, if there's a better way to give the vite plugin access to manifest_data at dev time, let me know.

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[madeleineostoja]

I'm in favour of just fixing this weirdness no matter what, but I will say that I think this is a seperate issue to allowing remote functions outside of $lib and routes within your project source.

I feel like whitelisting "external" sources (eg: node_modules) and src should probably be handled differently

[Rich-Harris]

#14293 was merged, so I'll close this — thanks