Add slim prop to Form component

[hackel]

Is your feature request related to a problem? Please describe.

When upgrading to v4, I replaced a ValidationObserver using the slim prop with a Form component as a div. This worked fine, but broke the flexbox layout that was used on the form.

Describe the solution you'd like

If the Form component supported the slim prop (or :as="null") to make a renderless component, it would solve the layout issue.

Describe alternatives you've considered

I rewrote my component using useForm and that worked fine, but it would be nice not to have to do that extra work.

[logaretm]

You can give it an empty string, like as="" but allowing null should be allowed, I will add that.