Support `NODE_SYMBOLS` in `mrb_parser_dump`

#### Before this patch:

  ```terminal
  $ bin/mruby -v -e '%i[1]; %I[#{2}]'
  mruby 2.0.1 (2019-04-04)
  00001 NODE_SCOPE:
  00001   NODE_BEGIN:
  00001     node type: 87 (0x57)
  00001     node type: 87 (0x57)
  (snip)
  ```

#### After this patch:

  ```terminal
  $ bin/mruby -v -e '%i[1]; %I[#{2}]'
  mruby 2.0.1 (2019-04-04)
  00001 NODE_SCOPE:
  00001   NODE_BEGIN:
  00001     NODE_SYMBOLS:
  00001       NODE_STR "1" len 1
  00001     NODE_SYMBOLS:
  00001       NODE_STR "" len 0
  00001       NODE_BEGIN:
  00001         NODE_INT 2 base 10
  00001       NODE_STR "" len 0
  (snip)
  ```

mrbgems/mruby-compiler/core/parse.y

@@ -6945,6 +6945,11 @@ mrb_parser_dump(mrb_state *mrb, node *tree, int offset)
     dump_recur(mrb, tree, offset+1);
     break;
 
+  case NODE_SYMBOLS:
+    printf("NODE_SYMBOLS:\n");
+    dump_recur(mrb, tree, offset+1);
+    break;
+
   case NODE_SELF:
     printf("NODE_SELF\n");
     break;