Add cpp_library_set_version for git-based versioning

[sean-parent]

Introduces the cpp_library_set_version() function to set project version variables from git tags after project() is called, enabling git-based versioning for custom setups. Updates documentation with usage instructions and refactors cpp_library_setup() to use the new function for version management.

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Note

Adds cpp_library_set_version for git tag-based project versioning and updates cpp_library_setup to use it; documents usage in README.

• CMake:
  • Versioning: Introduces cpp_library_set_version() to set PROJECT_VERSION* from git tags (respects CPP_LIBRARY_VERSION, strips v prefix, falls back to 0.0.0).
  • Refactor: cpp_library_setup() now calls cpp_library_set_version() and propagates PROJECT_VERSION* instead of duplicating parsing logic.
• Docs:
  • Adds API reference, usage examples, and guidance for cpp_library_set_version() in README.md (including stlab example and when-to-use notes).

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