Add encoding data to support core MIME/Unix/Windows encodings, including core

ISO 8859 sets (Latin 1-5, Cyrillic, Arabic, Greek, Hebrew), various EUCs, and common CJK encodings.

Data is hard-coded in order to achieve a hermetic runtime, and to simplify the build process. Data is in little-endian byte order. Trying to decode/encode CJK or other code-mapped encodings on a big-endian platform will result in a runtime InvalidArgumentError ("Could not create converter for...").

PiperOrigin-RevId: 221478869