Support `\hphantom` & `\vphantom`

The `\hphantom` & `\vphantom` are very useful in some cases.
For example, the vectors:

```latex
\vec{a\vphantom{b}}\cdot\vec{b}
```


The radicas

```latex
\sqrt{a\vphantom{b}} \cdot \sqrt b \cdot \sqrt{c\vphantom{b}}
```