Finish chapter 6

java/com/craftinginterpreters/lox/Expr.java

@@ -1,12 +1,13 @@
 package com.craftinginterpreters.lox;
 
-import java.util.List;
-
 abstract class Expr {
   interface Visitor<R> {
     R visitBinaryExpr(Binary expr);
+
     R visitGroupingExpr(Grouping expr);
+
     R visitLiteralExpr(Literal expr);
+
     R visitUnaryExpr(Unary expr);
   }