This PR was merged into the main branch.

Discussion
----------

Deprecate the package

I don't think there is any interest in _maintaining_ it anymore. Because of Jest and Babel, installing ``@symfony`/stimulus-testing` installs [**~500 subdependencies**](https://npmgraph.js.org/?q=%40symfony%2Fstimulus-testing), which is totally unacceptable, since the added-value of this package is only these two functions:
```js
export function mountDOM(html = '') {
    const div = document.createElement('div');
    div.innerHTML = html;
    document.body.appendChild(div);

    return div;
}

export function clearDOM() {
    document.body.innerHTML = '';
}
```

It's like a Composer package, but all its dependencies being shadowed and not removable by the final user.

Removing it from `@symfony`/ux, where we don't use Jest and Babel but Vitest, allowed us to remove ~400 useless Node.js dev dependencies: symfony/ux#2879

The package is still experimental, and I suggest to deprecate this package. I've added migration steps for people being stuck with it, even if I think we, Symfony, are maybe the only ones to use it:

https://github.com/user-attachments/assets/86a21c37-8560-43dc-bec9-0043875831ea

We can observe:
1. it always had ~1k downloads per week this last year,
2. more recently we had ~21k downloads per week, and I believe that's correlated to my last two-months work on UX
3. and since a few days, it started to drop again (currently at 8k downloads for the last full week), when I removed it from UX

Commits
-------

07d2209 Deprecate the package