New meson.global_build_root() and meson.global_source_root() methods

Returns the root source and build directory of the main project.

Those are direct replacement for meson.build_root() and meson.source_root() that have been deprecated since 0.56.0. In some rare occasions they could not be replaced by meson.project_source_root() or meson.current_source_dir(), in which case the new methods can now be used instead. Old methods are still deprecated because their names are not explicit enough and created many issues when a project is being used as a subproject.