refactor: modularize CMakeLists.txt for better maintainability

refactor: modularize CMakeLists.txt for better maintainability

- Split monolithic 235-line CMakeLists.txt into focused modules
- Main file reduced to 78 lines with clear section organization
- Created 5 specialized modules:
  * ZstdVersion.cmake - CMake policies and version management
  * ZstdOptions.cmake - Build options and platform configuration
  * ZstdDependencies.cmake - External dependency management
  * ZstdBuild.cmake - Build targets and validation
  * ZstdPackage.cmake - Package configuration generation

Benefits:
- Improved readability and maintainability
- Better separation of concerns
- Easier debugging and modification
- Preserved 100% backward compatibility
- All existing build options and targets unchanged

The refactored build system passes all tests and maintains
identical functionality while being much easier to understand
and maintain.