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.tres

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+[gd_resource type="Theme" load_steps=4 format=2]
+
+[ext_resource path="res://addons/godot_xterm/resources/xrdb_theme.gd" type="Script" id=1]
+[ext_resource path="res://addons/godot_xterm/themes/fonts/hack/hack_regular-3.003.ttf" type="DynamicFontData" id=2]
+
+[sub_resource type="DynamicFont" id=1]
+size = 39
+extra_spacing_bottom = 1
+font_data = ExtResource( 2 )
+
+[resource]
+Terminal/colors/Background = Color( 0.156863, 0.156863, 0.156863, 1 )
+Terminal/colors/Black = Color( 0, 1, 0.4, 1 )
+Terminal/colors/Blue = Color( 0, 1, 0.4, 1 )
+Terminal/colors/Cursor = Color( 0, 1, 0.4, 1 )
+Terminal/colors/Cyan = Color( 0, 1, 0.4, 1 )
+"Terminal/colors/Dark Grey" = Color( 0, 1, 0.4, 1 )
+Terminal/colors/Foreground = Color( 0, 1, 0.4, 1 )
+Terminal/colors/Green = Color( 0, 1, 0.4, 1 )
+"Terminal/colors/Light Blue" = Color( 0, 1, 0.4, 1 )
+"Terminal/colors/Light Cyan" = Color( 0, 1, 0.4, 1 )
+"Terminal/colors/Light Green" = Color( 0, 1, 0.4, 1 )
+"Terminal/colors/Light Grey" = Color( 0, 1, 0.4, 1 )
+"Terminal/colors/Light Magenta" = Color( 0, 1, 0.4, 1 )
+"Terminal/colors/Light Red" = Color( 0, 1, 0.4, 1 )
+"Terminal/colors/Light Yellow" = Color( 0, 1, 0.4, 1 )
+Terminal/colors/Magenta = Color( 0, 1, 0.4, 1 )
+Terminal/colors/Red = Color( 0, 1, 0.4, 1 )
+Terminal/colors/White = Color( 0, 1, 0.4, 1 )
+Terminal/colors/Yellow = Color( 0, 1, 0.4, 1 )
+Terminal/fonts/Bold = null
+"Terminal/fonts/Bold Italic" = null
+Terminal/fonts/Italic = null
+Terminal/fonts/Regular = SubResource( 1 )
+script = ExtResource( 1 )

themes/retro_green.xresources

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+! 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

themes/retro_green.xresources.import

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+[remap]
+
+importer="godot_xterm_xrdb_importer"
+type="Theme"
+path="res://.import/retro_green.xresources-3110df277701da17f502f808acb29c51.res"
+
+[deps]
+
+source_file="res://themes/retro_green.xresources"
+dest_files=[ "res://.import/retro_green.xresources-3110df277701da17f502f808acb29c51.res" ]
+
+[params]
+