Add base64url encode/decode

**Please describe your feature request.**
base64 has a variant commonly referred to as base64url that replaces `+` and `/` with `-` and `_`

**Describe the solution you'd like**
Add `@base64url`/`@base64urld` functions/operators.

**Describe alternatives you've considered**
Process the base64url stuff outside yq.

**Additional context**
https://datatracker.ietf.org/doc/html/rfc4648#section-5
