feat: add difficulty levels, question limit, and clean division

[MChand20656789]

What I changed

• Added three difficulty levels: easy (1–10, + and - only),
  medium (1–20, all operators), hard (1–50, all operators)
• Added a question limit so players choose how many questions
  to answer instead of playing until they get one wrong
• Added a final score percentage at the end of each game
• Fixed division to always produce whole number answers —
  previously could generate unfair decimals like 2.333...
• Removed redundant type check in error handling logic
• Added README.md with usage instructions and difficulty level table

Why

These changes make the game more replayable, fairer, and beginner-friendly.