feat: Support mmultiscripts.

[fukatani]

I want mmultiscripts for my application.

testcase:

<math>
    <mn>2</mn>
    <mmultiscripts>
        <mi>X</mi>
        <mi>d</mi>
        <mi>c</mi>
        <mprescripts />
        <mi>b</mi>
        <mi>a</mi>
    </mmultiscripts>
    <mn>1</mn>
</math>