feat: support icon as textfield prefix/suffix

[vsandvold]

This PR adds a specialized kind of Affix component, that renders an icon as prefix/suffix in a Textfield.

I'm happy to discuss the interface for this new component. It would also be possible to split out the existing Affix into separate LabelAffix and ButtonAffix component, possibly making the code a bit clearer. Please let me know if you are open to discuss this idea, or if you prefer to incorporate icons in a different way (if at all).

[benja]

Let's discuss this in tomorrow's meeting? I think it'd fit perfectly in the combobox discussion.

[vsandvold]

Please feel free to close this PR is you have decided not to accept the proposed changes. I think you should consider it though, so let me try to explain 😄

When I was working on the Affix component, I noticed if-else blocks that toggle different kinds of behavior inside the component. Along the lines of "if this prop, then render this kind of content, and if that prop, render some other kind of content". Components (and classes) with conditional behavior is a kind of code smell, and to me this looks like an opportunity to introduce some polymorphism. With a few, more specialized components, the user of the component can decide what kind of (specialized) Affix is needed, and the components themselves can focus on separate concerns (ie. render a label, or a button, or an icon). If you want to re-use some common Affix behavior across these specialized components, you can do this with composition. It also becomes easier to extend the Affix component with more specialized behavior when it is split like this.

If you want to make this change, you could perhaps introduce the new components while deprecating the old Affix, and then remove it in a future major version release?

But then again, this is just a suggestion. I have currently solved then problem by creating a custom IconAffix component in our own codebase, and using that together with the Fabric Combobox and TextField components. That works fine as long as the Affix interface remain the same in Fabric, but it would be better for us to have this implemented directly in Fabric, of course.

I hope that was useful!