feat: command \center now has an option to center its content vertically

classes/plain.lua

@@ -310,10 +310,14 @@ function class:registerCommands ()
     SILE.settings:set("linebreak.tolerance", 10000)
   end)
 
-  self:registerCommand("center", function (_, content)
+  self:registerCommand("center", function (options, content)
     if #SILE.typesetter.state.nodes ~= 0 then
       SU.warn("\\center environment started after other nodes in a paragraph, may not center as expected")
     end
+    if SU.boolean(options.vertical) then
+      SILE.call("hbox")
+      SILE.call("vfill")
+    end
     SILE.settings:temporarily(function()
       SILE.settings:set("current.parindent", 0)
       SILE.settings:set("document.parindent", 0)

tests/centering-vertically.expected

@@ -0,0 +1,47 @@
+Set paper size 	595.275597	841.8897729
+Begin page
+Mx 	49.7638
+My 	42.0945
+Mx 	160.4430
+My 	288.5648
+Set font 	Gentium Plus;10;400;;normal;;;LTR
+T	55 43 44 54 w=19.7852	(THIS)
+Mx 	182.8707
+T	51 43 53 36 54 40 w=33.0518	(PHRASE)
+Mx 	218.5650
+T	54 43 50 56 47 39 w=34.5264	(SHOULD)
+Mx 	255.7340
+T	37 40 w=10.4590	(BE)
+Mx 	268.8356
+Mx 	4.9609
+Mx 	6.1523
+Mx 	5.8008
+Mx 	5.3613
+Mx 	5.5078
+Mx 	4.0820
+Mx 	5.9424
+T	40 a=4.9609 59 a=6.1523 36 a=5.9521 38 a=5.3613 55 a=5.5078 47 a=4.7607 60 a=5.9424	(EXACTLY)
+Mx 	309.2857
+Mx 	5.4639
+Mx 	5.5078
+T	36 a=5.9521 55 a=5.5078	(AT)
+Mx 	322.9000
+T	55 43 40 w=17.0068	(THE)
+Mx 	342.5494
+T	48 44 39 39 47 40 w=32.6172	(MIDDLE)
+Mx 	377.8092
+T	50 41 w=10.7813	(OF)
+Mx 	391.2330
+T	55 43 40 w=17.0068	(THE)
+Mx 	410.8824
+Mx 	4.6387
+Mx 	5.8008
+Mx 	5.8301
+Mx 	4.9609
+T	51 a=5.2295 36 a=5.9521 42 a=5.8301 40 a=4.9609	(PAGE)
+T	4 w=2.7197	(!)
+Mx 	295.2916
+My 	780.6177
+T	20 w=4.6924	(1)
+End page
+Finish

tests/centering-vertically.sil

@@ -0,0 +1,3 @@
+\begin{document}
+\center[vertical=true]{THIS PHRASE SHOULD BE EXACTLY AT THE MIDDLE OF THE PAGE!}
+\end{document}