Keyboard handling is not sufficient for all terminal usage

[mmastrac]

The current keyboard approach works somewhat, but as @j4james pointed out in #6, this might not be sufficient for more advanced editing tasks.

Right now we emulate ctrl/shift/etc presses as needed to get the correctly-modified keypresses to match the user's OS inputs. For this emulation to be more accurate, we probably need to move to a closer mapping between the emulator keyboard state and OS keyboard state.

I'm open to suggestions here, especially if there is some prior art in other emulators that handle this well!

[Terri-Kennedy]

If possible, it would be good to also support the PC-interface LK4xx keyboards. Some of those keys make it through Windows (like the Do key) but I think the standard keyboard driver may filter some of the other extra keys.

[mmastrac]

@Terri-Kennedy, good idea. I was also considering allowing for a serial interface for the keyboard driver so you could just use a USB<->serial adapter without any OS interference.