Make a leaderboard and score entry functionality for high scores

[2021rkim]

This is not necessary, but it would be a nice thing to set-up, even if we don't have any kind of connection to a server (since this can all be hosted locally). I'm not sure how text input works for SMBX, but it would be good to make some kind of leaderboard that shows up before a round is started so that ppl can feel validated for playing the game :)