feat: add index signature inference when using `passthrough` on ZodObjects

[santosmarco-caribou]

Overview

As we know, the passthrough option allows any key to, as the name suggests, pass through the parser.

This PR adds a { [k: string] any } index signature on the Zod Objects marked as passthrough. (because that's what happens in reality, right?)

If you declare a catchall, though, the catchall has priority over it (as says the docs and the code) so the inference caused by the passthrough option goes away.

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[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.