LibCMake: Treat block, endblock and return as control keywords

I missed these before because they weren't mentioned on the language
description page of the CMake manual.

Userland/Libraries/LibCMake/Token.h

@@ -33,10 +33,13 @@ enum class ControlKeywordType {
     EndWhile,
     Break,
     Continue,
+    Return,
     Macro,
     EndMacro,
     Function,
     EndFunction,
+    Block,
+    EndBlock,
 };
 
 struct Token {