Change UNDEF Node to hold their items to keep the original grammar
structure.

For example:

```
undef a, b
```

Before:

```
@ NODE_BLOCK (id: 4, line: 1, location: (1,6)-(1,10))*
+- nd_head (1):
|   @ NODE_UNDEF (id: 1, line: 1, location: (1,6)-(1,7))
|   +- nd_undef:
|       @ NODE_SYM (id: 0, line: 1, location: (1,6)-(1,7))
|       +- string: :a
+- nd_head (2):
    @ NODE_UNDEF (id: 3, line: 1, location: (1,9)-(1,10))
    +- nd_undef:
        @ NODE_SYM (id: 2, line: 1, location: (1,9)-(1,10))
        +- string: :b
```

After:

```
@ NODE_UNDEF (id: 1, line: 1, location: (1,6)-(1,10))*
+- nd_undefs:
    +- length: 2
    +- element (0):
    |   @ NODE_SYM (id: 0, line: 1, location: (1,6)-(1,7))
    |   +- string: :a
    +- element (1):
        @ NODE_SYM (id: 2, line: 1, location: (1,9)-(1,10))
        +- string: :b
```