Ports the zMUD/CMUD numpad.set convention: plain numpad for normal
movement, Ctrl+numpad for moving up a level in that direction, and
Ctrl+Alt+numpad for moving down a level, all sent as literal MUD
commands via Accelerator. Alt+Enter toggles the whole set on/off (with
a sound + spoken confirmation) by clearing/restoring the accelerators,
so a disabled numpad types normally instead of doing nothing.
Cleared the numbers/*/-/+ entries in the world's native <keypad> block
since they'd otherwise double-send alongside the new Accelerator-based
bindings - left "." and "/" (hide/inventory) alone since those aren't
part of this feature. Documented the full three-layer key list and the
toggle in the readme.
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