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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Latest commit: 97466409192b46d726123065b606e74b37c59a9e

The changes in this PR will be included in the next version bump.

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@sveltejs/kit	Minor

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[teemingc]

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

[dummdidumm]

Related #12480

[rChaoz]

Should I bring this up-to-date? Or maybe can this be considered to become the default in v3?

[Rich-Harris]

I think it would make sense for this to be the default in v3 — it's probably not worth adding a new option in the meantime, and since this PR has a lot of merge conflicts I'll go ahead and close it. I've given #12732 a 3.0 milestone to ensure it doesn't get left out. Thanks!