Add support for XTerm's NumLock mode

[j4james]

Summary of the Pull Request

NumLock mode provides a way to alter the behavior of the numeric keypad,
switching between the standard DEC interpretation of DECKPAM, and a
kind of Linux-compatible mode, where DECKPAM only applies if NumLock
is off. Even then it only applies to the non-digit keys, since the digit
keys are interpreted as cursor keys when NumLock is off.

References and Relevant Issues

#16506, #16511, #16654, #16675

Detailed Description of the Pull Request / Additional comments

Validation Steps Performed

PR Checklist

• Closes #xxx
• Tests added/passed
• Documentation updated
  • If checked, please file a pull request on our docs repo and link it here: #xxx
• Schema updated (if necessary)

[j4james]

I don't particularly like this as a concept, and would prefer a simple setting that could disable DECKPAM, but I thought it worth sharing the code in case anyone else thinks it's a good idea.