refactor handling of plugin loading to make plugin a global property

src/library.h

@@ -140,6 +140,7 @@ void lammps_close(void *handle);
 void lammps_mpi_init();
 void lammps_mpi_finalize();
 void lammps_kokkos_finalize();
+void lammps_plugin_finalize();
 void lammps_python_finalize();
 
 void lammps_error(void *handle, int error_type, const char *error_text);