decodeBase64
Decodes this string using the standard base64 alphabet of RFC 4648 section 4.
Both padded and unpadded input is accepted. When padding is present, the total length must be a multiple of four and at most two = characters may appear, only at the end. An empty string produces an empty array. Trailing bits that do not form a full byte are discarded without validation, so non-canonical final characters are accepted.
Return
the decoded bytes.
Throws
if the input contains a character outside the standard alphabet, if padding is malformed, or if the number of data characters leaves a remainder of one when divided by four.