The behaviour of $tab sole-effects lacked a 'tab' key, so Soto wasn't
catching or writing them to the screen. This commit amends both the
handler and the sole-effect to fix that behaviour.
The keypress handler would check if backspace was past position 0
before deleting one character at that position. If you were at position
0, then it would drop down to the 'else' and send the key value as a
string, throwing off the state between front and back end. This commit
fixes the issue, ignoring Backspace at position 0.
