Set z-index of last used slider handle

[jjmax75]

I'm using this on a project we're currently building.

If it's something you'd like to integrate into the master branch let e know and I'll do some refactoring to make this feature optional.

Reason behind change -
We've given each handle a 44px square touchzone.
Sometimes the handles' touchzone overlaps on the range slider. We want our users to be able to select the last used handle in that case.

Note -
Also needed to modify eslint rules to allow setting styles when looping through child nodes.

[jjmax75]

I'm not sure why the checks are failing, let me know if you're interested in this feature I'd be happy to work through any problems with my code if you think other users would benefit