[ruby] Implement clear and actually use builtin pwd instead of /bin/pwd

ruby/shell/builtins.rb

@@ -66,9 +66,14 @@ module Shell
       0
     end
 
-    def bulitin_pwd(_args)
+    def builtin_pwd(_args)
       puts Dir.pwd
       0
     end
+
+    def builtin_clear(_args)
+      print "\033[2J"
+      0
+    end
   end
 end