feat: support server-only and client-only modules (#2162)

[YanAnghelp]

Closes #2162

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Latest commit: 54fc19b

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

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@solidjs/start	Minor

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commit: 54fc19b

[brenelz]

this is all that is needed?

[YanAnghelp]

Yes, I have already tested it.

[YanAnghelp]

There's a known limitation: when a client component imports a file that uses server-only, even if all functions in that file are marked with "use server", the build will incorrectly treat this as the client component directly calling server-only functions and throw an error. The only workaround is to apply "use server" at the file level.

However, "use server" already guarantees that the functions execute on the server — in that case, server-only is redundant. server-only is primarily meant to protect functions that must run on the server and cannot be called directly from outside (i.e., not exposed as RPC endpoints).