Emit `class` attributes in MathML Output

Description

Currently, I noticed that multiline-equation was added in MathML output classes and used in CSS selectors. For styling purposes. I propose that gives each typst element a unique class when render into MathML. like a math.mat() might produce

<mrow class="matrix">
    <mo>(</mo>
    <mtable>
        ...
    </mtable>
    <mo>)</mo>
<mrow>

Use Case

If not. It is not very easy to apply a custom css style for matrix. e.g. if I'd like to adjust the rows gap and columns gap of a matrix, I have to set

math mrow > mtable mtr:not(:last-child) mtd {
  padding-bottom: 0.2em !important;
}

math mrow > mtable mtr mtd:not(:last-child) {
  padding-right: 0.5em !important;
}

currently. Note that the !important is to prevent these styles from overloading by the Typst default MathML stylesheets. To be specific, the following styles are designed to insert a lead between multiline equations. But it will effect the metrics within a multiline equation.

math > mtable.multiline-equation mtr:not(:last-child) mtd {
  padding-bottom: 0.5em;
}

My current approach is not quite robust(e.g. cases cannot get excluded). if we have a unique class name, it will be much easier select matrix.

Start by locating the MathML output implementation and the handling of math.mat() and multiline-equation. Determine how Typst elements are represented in the emitted MathML, then define consistent unique class names for the relevant elements and verify that matrix-specific CSS selectors can target them without affecting cases or multiline equations.