This function creates a "LaTeXpackage" object
that provides support for the LaTeX zref package.
This allows commands like \zsavepos{label}
to be included in LaTeX snippets when calling
grid.latex.
A new command, \xdvirzmark{label}, is defined
to allow saved positions also to be recorded in the rendered LaTeX in
R. This produces nullGrob objects at the
relevant locations plus anchors (for justification) at the relevant
locations.
A predefined package is pre-registered
under the name "zref".