feat: add base64 string validation

[morinokami]

Hi, I noticed there was no validation action for Base64 strings, so I added it. Thanks!

[fabian-hiller]

Thank you very much! I will try to review and merge it as soon as possible, but don't expect a quick response as I just released Valibot v0.31.0 and should be working on other things first.

As a workaround you can use the regex with our regex action: https://valibot.dev/api/regex/