Updated the Human controlled Snake game - #29

Added following features and some basic functionalities to enhance the user experience:

• Pause/Resume Functionality: Now the player can pause and resume mid-game with a key (Space).
• Game Over Screen: A proper game over screen is shown to the user instead of immediately closing the game, and it also displays the final score and and allows replay without restarting the program.
• Difficulty Levels: Added progressive difficulty feature, each time you eat food (Score + 1), the snake moves slightly faster. The formula is: Current Speed = 20 + (Score * 0.5).
• Reverse Direction Prevention: Added is_valid_direction() method that prevents the snake from moving directly opposite to its current direction, If the snake is moving RIGHT, it can't immediately turn LEFT (and vice versa for UP/DOWN).
• On-Screen Instructions: Added instruction text at the bottom of the game screen,
  Displays: "Arrow Keys: Move | SPACE: Pause | ESC: Quit".

FLOW

1. Start Screen - Shows controls and waits for any key press
2. Gameplay - Clean UI with only current score and speed
3. Game Over Screen - Shows final score and high score for comparison
4. Restart or Quit