decodes a base64 string, decoding is strict as required by RFC 4648
non alphabet characters, line breaks, missing padding cause errors

NYI: decoding does currently not reject encodings where the padding bits have not been set to zero prior to encoding
therefore in some cases multiple encodings can be decoded to the same data
See RFC4648 section 3.5: https://datatracker.ietf.org/doc/html/rfc4648#section-3.5