feat: add error fallback option in Encoding.convert

[polygonplanet]

Resolve #43

Added a new { fallback: 'error' } option to Encoding.convert.
This option throws an error if a character cannot be converted, allowing for explicit error handling.

Example:

const unicodeArray = Encoding.stringToCode('おにぎり🍙ラーメン🍜');
try {
  const sjisArray = Encoding.convert(unicodeArray, {
    to: 'SJIS',
    from: 'UNICODE',
    fallback: 'error' // Specify 'error' to throw an exception
  });
} catch (e) {
  console.error(e); // Error: Character cannot be represented: [240, 159, 141, 153]
}