Implement dom_document_type_get_(public|system)id.
Implement dom_document_type_get_(public|system)id.

prepare/netsurf_libdom,dunkelstern/libdom,Achal-Aggarwal/libdom,dunkelstern/libdom,Achal-Aggarwal/libdom,prepare/netsurf_libdom,prepare/netsurf_libdom,Achal-Aggarwal/libdom,dunkelstern/libdom,prepare/netsurf_libdom,prepare/netsurf_libdom,Achal-Aggarwal/libdom,Achal-Aggarwal/libdom,dunkelstern/libdom,dunkelstern/libdom,prepare/netsurf_libdom,Achal-Aggarwal/libdom,dunkelstern/libdom