Added main gameplay

Added enemies:

    Snowflake (appears from the very beginning, then every 5 seconds, moderately falls down)

    Fireworks (appears 5 seconds after the start, then every 10 seconds, quickly flies up)

    Snowman (appears 20 seconds after the start, then every 30 seconds, slowly goes from one edge of the map to the other)

    Snowball (appears 35 seconds after the start, then every 10 seconds, flies along a random trajectory)

The task is not to get behind the purple walls and survive until the ball reaches the top of the Christmas tree