Make the resulting form more amenable to normal DOM operations.

This is purely an aesthetic choice:

* The DOM `reset()` function can be used to clear the form, but can't be used to clear editable DIVs. Binding the editable div to a (hidden) form field allows `reset()` to clear both.

* We can find the target `form` element out of the event, but the API needed to do so differs between events dispatched to form controls and events dispatched to random DOM nodes. Using `closest('form')` works for both kinds of event target.

In practice, there is little need to make sure the message input form uses "normal" DOM APIs for functional reasons. Everything inside `MessageInput` is controllable through the component's script. This change isn't based on a functional need, but rather in the hopes that integrating with the DOM APIs makes it easier for _code we don't control_ - screen readers, password managers, saved-form support in browsers, &c - to integrate with Pilcrow. It is purely speculative.

(This also used to be necessary because Firefox didn't support `contenteditable="plaintext-only"`, but [support was added in March][ff-pto]

[ff-pto]: https://www.mozilla.org/en-US/firefox/136.0/releasenotes/#:~:text=The%20value%20plaintext%2Donly%20can%20now%20be%20specified%20for%20the%20contenteditable%20attribute%2C%20making%20the%20raw%20text%20of%20an%20element%20editable%20but%20without%20supporting%20rich%20text%20formatting.

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