[slider] Allow Slider.Indicator to be dragged

## Summary

Allow the `Slider.Indicator` component to be dragged when there are two thumbs. Not sure how this would work for other number of thumbs.

## Examples in other libraries

Here is a good example from [Syncfusion Flutter widgets](https://help.syncfusion.com/flutter/range-slider/drag-mode):

## Motivation

Having this built-in would improve user experience significantly. If this is already possible without significant configuration, I would love to see an example in the docs.