[react-native-slider] Multi-thumb option

[https://github.com/react-native-community/react-native-slider/issues/25](https://github.com/react-native-community/react-native-slider/issues/25)

Begin with the linked react-native-slider issue, since this board entry provides no implementation files or tests. Define the required multi-thumb behavior and acceptance criteria there before identifying the affected code and tests; completion should be a reviewed implementation of the requested option.

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