build, add Conan 2.x recipe for ConanCenter

[steven-varga]

Add Conan 2.x support so enterprise C++ consumers can install h5cpp-compiler via conan install h5cpp-compiler or as a tool_requires.

Scope

• Add conanfile.py with CMakeToolchain + CMakeDeps
• Add test_package/ validation
• CI job that runs conan create on ubuntu-24.04, macos-15, windows-latest
• README update with Conan instructions

Blockers

• ConanCenter llvm-core is at 19.1.7; project targets LLVM 20. May need version alignment.