Support DEC Private Modes (DECSET,DECRST,DECRQM).

[Jeff Quast]

There are many terminal features that can be probed (DECRQM), enabled (DECSET), and disabled (DECRST) for support.

We should have a blessed function to toggle and request these modes by their digits, and provide an enumerator for the probe response (0: 'NOT RECOGNIZED', 1: 'SET', 2: 'RESET', 3: 'PERMANENTLY SET', 4: 'PERMANENTLY RESET')

Two very large lists of these modes,

• https://terminalguide.namepad.de/seq/
• https://chromium.googlesource.com/apps/libapps/+/master/hterm/doc/ControlSequences.md#decset

See also,

• https://gist.github.com/christianparpart/d8a62cc1ab659194337d73e399004036
• https://github.com/mintty/mintty/wiki/CtrlSeqs
• #7