Replace deprecated and vulnerable package

This replaces `@gcornut/valibot-json-schema` with `@valibot/to-json-schema` to address https://github.com/advisories/GHSA-vqpr-j7v3-hqw9, as described in issue https://github.com/ciscoheat/sveltekit-superforms/issues/662 This PR still has failing tests, as the package in question is not a drop-in replacement, and needs further implementation.