Support Bootstrap 5 alpha 3 floating labels

[thimo]

Bootstrap 5 alpha 3 supports floating labels: https://v5.getbootstrap.com/docs/5.0/forms/floating-labels/

It would be nice to enable this in bootstrap_form with an option, something like <%= f.text_field :query, floating: true %>.

To enable this a few things must be done in the rendering:

1. use form-floating as the control class
2. revert order of input and label elements
3. add placeholder to input field, fill with label value by default

<div class="form-floating mb-3">
  <input placeholder="Search" class="form-control" type="text" name="filter[query]" id="filter_query">
  <label for="filter_query">Search</label>
</div>

I would love to pick this up, but if anybody else has the time, please feel free to do so. Would also love feedback on how to enable this functionality, is floating: true the right way to do that?

[lcreid]

Thanks for proposing this. We definitely want to support floating labels. (We want to support whatever Bootstrap supports.)

We'd be very interested in seeing a PR if you want to take a crack at it. When we see how it works I think we can have better feedback for you. Feel free to ask questions here if you're wondering about existing code.

Thanks for your interest and input in bootstrap_form.

[lcreid]

Closed by #584 .