Add external JSON5 library for three-way benchmark comparison

[Copilot]

This PR extends the benchmark module to include a third JSON5 library (at.syntaxerror.json5:2.1.0) for comprehensive performance comparison across three different implementations:

1. JSON5 (this project) - Uses kotlinx.serialization
2. JSON (kotlinx.serialization) - Standard JSON baseline
3. External-JSON5 (at.syntaxerror.json5:2.1.0) - External JSON5 library

Changes Made

• Added dependency: at.syntaxerror.json5:2.1.0 to benchmark module
• Extended SerializationBenchmark.kt: Added support for the external JSON5 library with proper serialization/deserialization benchmarking
• Updated BenchmarkRunner.kt: Enhanced reporting to show all three libraries in both console output and generated reports
• Updated tests: Modified BenchmarkTest to expect 6 results (3 libraries × 2 operations) instead of 4
• Updated documentation: Enhanced benchmark README.md to reflect the three-way comparison

Sample Results

The benchmark now provides comprehensive performance analysis:

SimplePerson Serialization: JSON5=0.031ms, JSON=0.010ms, External-JSON5=0.012ms
ComplexPerson Serialization: JSON5=0.076ms, JSON=0.021ms, External-JSON5=0.028ms
Company Serialization: JSON5=0.240ms, JSON=0.030ms, External-JSON5=0.044ms

Performance Ranking: JSON (fastest) > External-JSON5 > JSON5 (this project)

Key Insights

• kotlinx.serialization JSON remains the performance leader
• External JSON5 library provides good balance of JSON5 features with reasonable performance
• This project's JSON5 offers seamless kotlinx.serialization integration but at a performance cost

The benchmark results help users choose the right library based on their priorities: performance (JSON), JSON5 features with good performance (External-JSON5), or kotlinx.serialization integration (this project).

Fixes #33.

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