Add support for Bootstrap Icons on floating label forms

[mdo]

Biggest concern would be that this has specific HTML requirements (icon must come first), but we already have that with this component so it might not be a huge deal. I made it so that the .form-floating-icon was a wrapper vs the the actual SVG/image so that placement becomes super easy (center center via flexbox).

[alecpl]

To make sure it works with selects maybe add one to the example.

I also wonder whether it would make sense to share some code/structure/class-names with the solution from #31667.