Improved Game Over Popup Appearance and Functionality

This is the PR for the issue #3

Introduces a visually appealing popup for the game-over screen.
The popup features a centered layout with a semi-transparent background, rounded corners, and a subtle shadow for a modern look.
Displays the final score and old high score achieved by the player in the popup.
Displays a "Congratulations!" message if you score a High Score.
The "Play Again" button is styled with a bold red color to make it more prominent and inviting.
Enhances the overall user experience and adds a touch of polish to the game interface.

iiitl / snake-game-js