Allow setting Japanese fonts when using LuaLaTeX (#3873)

...by using the `luatexja-fontspec` and `luatexja-preset` packages. Use
existing `CJKmainfont` and `CJKoptions` template variables. Add
`luatexjafontspecoptions` for `luatexja-fontspec` and `luatexjapresetoptions`
for `luatexja-preset`.

data/templates/default.latex

@@ -64,8 +64,21 @@ $else$
 $endif$
 $endif$
 $if(CJKmainfont)$
+  \ifxetex
     \usepackage{xeCJK}
     \setCJKmainfont[$for(CJKoptions)$$CJKoptions$$sep$,$endfor$]{$CJKmainfont$}
+  \fi
+$endif$
+$if(luatexjapresetoptions)$
+  \ifluatex
+    \usepackage[$for(luatexjapresetoptions)$$luatexjapresetoptions$$sep$,$endfor$]{luatexja-preset}
+  \fi
+$endif$
+$if(CJKmainfont)$
+  \ifluatex
+    \usepackage[$for(luatexjafontspecoptions)$$luatexjafontspecoptions$$sep$,$endfor$]{luatexja-fontspec}
+    \setmainjfont[$for(CJKoptions)$$CJKoptions$$sep$,$endfor$]{$CJKmainfont$}
+  \fi
 $endif$
 \fi
 % use upquote if available, for straight quotes in verbatim environments