Add support for a mixed base64 and base64url encoding?

[lucacasonato]

Elsewhere, @bakkot says that apparently Node.js's Buffer.from(x, "base64"), as well as CSP, support mixing base64 and base64 URL in the same string, when decoding. That might be a variant worth supporting in the API as well.

Originally posted by @domenic in #7 (comment)

[annevk]

The way CSP and SRI appear to do this (in WebKit anyway) this is by trying one and then the other. That's not something we want to endorse.

[bakkot]

Not having any reason that anyone would ever want this, we're not planning on doing this.