String `"."` in math becomes an `mn` in MathML export

### Description

From #8258, the Typst code
```typ
$ (e^x - e^(-x)) / 2 thin "." $
```
produces
```html
<math display="block">
  <mfrac>
    ...
  </mfrac>
  <mspace width="0.1667em"></mspace>
  <mn>.</mn>
</math>
```

### Operating system

_No response_

### Typst version

- [ ] I am using the latest version of Typst