Add retro term example scene

Uses:
- Larger (size 39) font for 80x24 rows.
- World environment with glow enabled.
- Retro green color scheme based on https://superuser.com/a/1206781
- CRT Shader by Peter Höglund (pend00), released under CC-0 license.
  Downloaded from https://godotshaders.com/shader/vhs-and-crt-monitor-effect/

themes/retro_green.xresources

@@ -0,0 +1,36 @@
+! special
+*.foreground:   #00ff66
+*.background:   #282828
+*.cursorColor:  #00ff66
+
+! black
+*.color0:       #00ff66
+*.color8:       #00ff66
+
+! red
+*.color1:       #00ff66
+*.color9:       #00ff66
+
+! green
+*.color2:       #00ff66
+*.color10:      #00ff66
+
+! yellow
+*.color3:       #00ff66
+*.color11:      #00ff66
+
+! blue
+*.color4:       #00ff66
+*.color12:      #00ff66
+
+! magenta
+*.color5:       #00ff66
+*.color13:      #00ff66
+
+! cyan
+*.color6:       #00ff66
+*.color14:      #00ff66
+
+! white
+*.color7:       #00ff66
+*.color15:      #00ff66