Before, keyboard input used key codes to map events to Apple II keys.
This worked reasonably well, but `event.keyCode` was deprecated and
slated to be removed.
The refactored code now uses `event.key` which returns the localized,
keyboard-mapped key that the user pressed, which may be a letter or a
"symbolic" key. This is then transformed into an Apple II key.
One side effect of the refactoring is that the keys now light up as
you type and that combinations of mouse clicks on modifiers and plain
keys will take the modifiers into account.
Before, if the browser window wasn't tall enough to show the whole
keyboard, using the arrow keys in the window would cause the page
to move as well. Now all key events that are sent to the keyboard
have `preventDefault()` called on them.
