Improvements to slider component needed for more complex sliders used for Agora Partial implementations in D16846.

In this diff, we show that the component improvements do not break existing slider implementations.

We do not use any of the new features here. But we split out the change because it's not directly related to implementing agora partials.

npm start, create a token, put an icon in, slider features still work (I have not been able to test slider input in react testing library)