This function creates a "LaTeXpackage" object
that provides support for the LaTeX fontspec package.
The font argument provides some convenience for setting the main
font to be used. The font can be the common name of
a system font or a complete path to the font file.
For variable fonts, use renderer="harfbuzz" (and
the "luahbtex" TeX engine).
For more complex situations, fontspec commands can be added to
the LaTeX code that is sent to functions like
author and grid.latex (see the Examples).
A predefined package, with no main font defined, is pre-registered
under the name "fontspec".