feat: z.string.cidr() - support CIDR notation

[wataryooou]

This PR supported CIDR notation. (This PR doesn't support IP address range.)
FYI: https://en.wikipedia.org/wiki/Classless_Inter-Domain_Routing

const cidr = z.string().cidr();
cidr.parse("192.168.0.0/24"); // pass
cidr.parse("2001:db8::/32"); // pass
cidr.parse("192.168.1.1/33"); // fail
cidr.parse("2001:db8::1/129"); // fail

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[colinhacks]

Is there an argument for making this z.string().cidr()?

[wataryooou]

@colinhacks

Is there are argument for making this z.string().cidr()?

That's a great idea 👍
I've made the changes accordingly. Could you review it once more?