feat: modernize simpleson with C++17, optimize performance, and add benchmark integration

[yeah14]

Summary

• Modernize simpleson core implementation in json.h and json.cpp with C++17-oriented cleanup (safer casts, exception handling improvements, clearer resource semantics, and related maintainability updates).
• Improve runtime performance in key paths including parsing, serialization, and object-key access while keeping external JSON semantics and usage largely compatible.
• Expand README.md with modernization notes, benchmark results, test/coverage instructions, and practical usage examples.
• Integrate baseline_benchmark_test into the root build and add robust dependency gating:
  • Benchmark targets are added through the top-level CMake flow.
  • If Google Benchmark is unavailable, benchmark targets are skipped gracefully.
  • Memory benchmark target is only built when allocounter target exists.
  • Windows runtime DLL copy behavior is preserved for runnable benchmark executables.

Why

• Increase code safety and long-term maintainability by replacing legacy patterns with modern C++ practices.
• Reduce overhead in common workloads and provide measurable evidence of improvement.
• Make benchmark/test workflows reproducible for reviewers and contributors without forcing hard dependency requirements.

Build & compatibility notes

• Added CMake option: SIMPLESON_BUILD_BASELINE_BENCHMARKS (default ON).
• Environments without benchmark dependencies can still configure and build the project successfully; missing dependencies now result in clear skip messages instead of configuration failure.

Test plan

• Configure project with CMake in a clean build directory.
• Build core library and existing tests successfully.
• Run jobject_semantics_test and verify all tests pass.
• Build and run benchmark targets where dependencies are available.
• Verify graceful skip behavior when Google Benchmark and/or allocounter is absent.