Allow step marker styling

## Describe the Feature
Step marker styling is currently not controllable, and renders as small black text.

## Possible Implementations
A simple passing through of a style prop e.g. stepMarkerStyle={{color: 'white'}} into the Text components used to indicate step markers

## Related Issues
None

Inspect the Text components used to indicate step markers and trace how slider props are passed into them. Add the requested stepMarkerStyle entry point so marker styling can be controlled, then verify that a color such as white changes the rendered marker text.

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