/// This the most notable one of the DOS Cyrillic code pages. It has the same
/// box drawing characters as code page 437, so it can be used for decoding
/// DOS-era ASCII + box drawing data.
///
/// [Index visualization](https://encoding.spec.whatwg.org/ibm866.html),
/// [Visualization of BMP coverage](https://encoding.spec.whatwg.org/ibm866-bmp.html)
///
/// This encoding matches the Windows code page 866.