Change the output directory of "pathstr_config.h" from PROJECT_SOURCE_DIR to CMAKE_CURRENT_SOURCE_DIR. Otherwise, when "CMakeLists.txt" is included by other projects, the header file cannot be found.
Add `const` to member function 'pathstr::as<T>()'
add `pathstr::set_is_dir()` to set a `pathstr` as a directory even it is not ended with '/'.