Validation

The game works, but it accepts anything: digits, symbols, the same letter twice. Try typing "7" or guessing "a" again after it has already been guessed. Nothing stops you.

In this problem, you will add guardrails:

1. validateGuess: check whether a guess is a valid single letter that has not been guessed before
2. processGuessValidated: wire validation into the guess handler so invalid input is rejected