Fix #8731: Revise end markers scheme

Handle end markers in parser instead of in Scanner, adding end markers
to the context free syntax.

Author: odersky

compiler/src/dotty/tools/dotc/ast/Trees.scala

@@ -369,6 +369,8 @@ object Trees {
       comment.map(putAttachment(DocComment, _))
       this
     }
+
+    def name: Name
   }
 
   /** A ValDef or DefDef tree */