Represents a struct for handling both encode and decode of strings
using a custom character set (charset). This struct provides methods to
encode and decode strings based on the provided charset.
decodes a given encoded string based on a specified charset, using 4-character
groups to restore the original bytes. Each character in the decode_str string
is mapped to an index in charset to form the decoded bytes.
Encodes a given input string into an encoded format using a specified character set (charset).
This function groups bytes in chunks of 3 and maps them into 4-character segments based on charset.