Support folding for <!DOCTYPE

[angelozerr]

Given this xml file which defines DOCTYPE with a lof ENTITY:

<!DOCTYPE opt [
  <!ENTITY size "short">			
  <!ENTITY ResolutionX "1920"> 		
  <!ENTITY ResolutionY "1080">		
  <!ENTITY LevelMeterX "752">				
  <!ENTITY LevelMeterY "1036">			<!-- 44 -->				
  <!ENTITY PipDisplayX "845">				
  <!ENTITY PipDisplayY "1058">			<!-- 22 -->				
  <!ENTITY MainMenuButtonX "1191">
  <!ENTITY MainMenuButtonY "1011">		<!-- 69 -->
  <!ENTITY ToolbarButtonLeftX "707">
  <!ENTITY ToolbarButtonRightX "1194">
  <!ENTITY ToolbarButtonY1 "912">		<!--168 -->
  <!ENTITY ToolbarButtonY2 "947">		<!-- 133 -->
  <!ENTITY ToolbarButtonY3 "982">		<!-- 98 -->
  <!ENTITY QuickslotX "750">
  <!ENTITY QuickslotY "991">			<!-- 89 -->
  <!ENTITY QuickslotPageX "706">
  <!ENTITY QuickslotPageY "999">		<!-- 81 -->
  <!ENTITY LoadingScreenBarY "908">
  <!ENTITY LoadingScreenHintX "235">	
  <!ENTITY LoadingScreenHintY "752">
  <!ENTITY LoadingScreenWindowX "238">
  <!ENTITY LoadingScreenWindowY "240">
  <!ENTITY LoadingScreenWindowWidth "1024">
  <!ENTITY LoadingScreenWindowHeight "512">
  <!ENTITY LoadingScreenTextWidth "1000">
  <!ENTITY LoadingScreenAnimBox "100">
  <!ENTITY LoadingScreenAnimX "916">
  <!ENTITY LoadingScreenAnimY "-200">
]>
<opt>
  
</opt>

It should be nice to fold the DOCTYPE.

This sample comes from redhat-developer/vscode-xml#716