Build Cipher Performance Benchmark in JavaScript

Description:
Build a performance benchmarking script in JavaScript to measure the encryption speed of Caesar vs Vigenère ciphers using the performance.now() API. This will help contributors analyze algorithm efficiency.

Expected Tasks:

• In the js-ciphers/ folder, create a new script benchmark.js
• Import or include both Caesar and Vigenère functions
• Measure execution time for:
  • Short input (e.g., 50 characters)
  • Medium input (e.g., 500 characters)
  • Large input (e.g., 5000 characters)
• Log results to the console or save in a benchmark_results.json file
• Add a section to README.md under “Benchmarks” describing the setup

Stretch Tasks:

• Visualize results with a basic HTML + chart (e.g., using Chart.js)
• Add an average of 10 trials per test case for better accuracy