fix: don't treat Object.prototype members as param matchers

[Nic-Polumeyv]

a matcher named toString passed validation via the prototype chain and blew up during matching. Same class as #16072.

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Install the latest version of @sveltejs/kit from b6874e3:

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Open in pkg.svelte.dev: https://pkg.svelte.dev/repos/kit/pr/16612

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