build, add vcpkg manifest and registry port

[steven-varga]

Add vcpkg support so Windows, Linux, and macOS users can install h5cpp-compiler via vcpkg install h5cpp-compiler.

Scope

• Add vcpkg.json manifest with LLVM+Clang dependency
• Add portfile for upstream vcpkg registry submission
• CI job that validates vcpkg install on ubuntu-24.04 and windows-latest
• README update with vcpkg instructions

Blockers

• vcpkg LLVM port is at 18.1.6; project targets LLVM 20. May need version alignment or custom registry.