is_guess_correct: Establish if guess is correct and set game state accordingly
Description
Compares the guess in game$guess (index from game$guess_count)
with the corresponding target word in game$target. If the guess is
equal to the target, game$game_won and game$game_over are
both set to TRUE.
Usage
is_guess_correct(game)
Arguments
game
'wordler' game object (as generated by
new_wordler).