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

Start by reading the existing Caesar and Vigenère functions in the js-ciphers/ folder, then run the new benchmark.js script with performance.now(). Verify that results are reported for 50-, 500-, and 5000-character inputs, and update README.md with a Benchmarks section describing the setup.

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