feat: add option to recursively consider server dirs in $lib

[rChaoz]

Closes #12732

This PR adds a new config option kit.files.nestedServerDirs that, when enabled, considers any modules inside any $lib/**/server/** directory a server module, rather than just $lib/server/**.

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[eltigerchino]

Instead of adding an option, can we instead make this the default? Would that be considered a breaking change?

[paoloricciuti]

Instead of adding an option, can we instead make this the default? Would that be considered a breaking change?

Yes because if for some reason I'm importing a module on the client from /server/nested/module.ts it will fail with this