Add support for `base64url` strings

[marvinruder]

Fixes #3711

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[marvinruder]

The new method should probably also be added to https://github.com/colinhacks/zod/blob/main/README.md#strings, but I didn’t want to predict the version number under which this feature may be released.

[rotu]

I like the spirit, but I'm not sure I like the implementation. If the idea is "parse, don't validate", then maybe it would be more on-brand to provide a transform to get at the represented binary data, rather than just validate the string unchanged. (of course the same goes for the existing z.string().base64() method)

[marvinruder]

I like the spirit, but I'm not sure I like the implementation. If the idea is "parse, don't validate", then maybe it would be more on-brand to provide a transform to get at the represented binary data, rather than just validate the string unchanged. (of course the same goes for the existing z.string().base64() method)

Parsing the encoded data into a buffer would be an interesting opt-in feature given that people are only interested in the binary data. My main use case for base64url at the moment is Web Authentication, and a popular library implementing that standard expects base64url-encoded strings in many of its interfaces, so I am only interested in validation, as the library already does the parsing and expects unparsed strings. It would be great if Zod would (continue to) offer validation-only processing of base64(url)-encoded strings.

[rotu]

lgtm!