Support @base64url, @base64urld

[apstndb]

I think@base64d with non-utf8 content is a one of notable feature of gojq.
In real world, there are many base64url in JSON fields.
Can't support @base64url and @base64urld even if they are not supported in jq?

References

• @base64d does not handle web-safe encoded base64 jqlang/jq#2262