DOM utils

Not sure if it's worth creating a new package for this but:

- `addClasses(element, ['class'], {signal})`
- `appendElement(element, child, {signal})`

The latter might be easier to use as:

```js
element.append(child);
onAbort(signal, element) // after I add support for .remove()
```

Maybe `abortable-dom`?

The issue names no files, tests, or entry points. Start by reviewing the existing abort-utils API and clarifying whether these DOM helpers belong in this package or a new one. Done means the package boundary and the proposed abortable DOM API have been agreed before implementation begins.

Written by the indexing model from the issue text.