created AST-generator class

src/main/java/com/craftinginterpreters/tool/GenerateAst.java

@@ -0,0 +1,11 @@
+package com.craftinginterpreters.tool;
+
+public class GenerateAst {
+    public static void main(String[] args){
+        if (args.length != 1){
+            System.err.println("Usage: generate_ast <output directory>");
+            System.exit(64);
+        }
+        String outputDir = args[0];
+    }
+}