Add Reveal.JS md version for making presentations with markdown

If it was trying to show the login screen (not sure it was) it was
totally blank.

This is after leaving the computer open, screen went blank, may have
gone into sleep mode but i don't think so, and i closed the lid.  When
i opened again and pressed a key it did seem to wake up, making the
Big Blue Button sounds of trying to reconnect, but nothing i could press
(especially the brightness buttons of course, with and without the function
key) nor closing the lid and opening it again worked, had to hold down the
power button with no feedback and hope, and it did restart and come back
with a screen.

configuration.nix

@@ -46,7 +46,7 @@
     kdePackages.sierra-breeze-enhanced
     keepassxc
     libreoffice-qt6-fresh
-    light
+    # light
     magic-wormhole
     meld
     mkcert
@@ -57,6 +57,7 @@
     php # does PHP 8.2 currently and there's php81, php83, and php84 alternatives
     python3
     recoll
+    reveal-md
     samba
     slack
     signal-desktop