Changable size of touchable area of the thumbImage

## Describe the Feature

With the possibility to add a thumb image, it would be good to make the whole image touchable. Actually you can only touch a little area in the center of the image.

## Possible Implementations

Just like react-native-slider it could be implemented in props as "thumbTouchSize".

## Related Issues

Locate the component code that handles thumbImage and its touch area, then review react-native-slider's thumbTouchSize behavior for comparison. Done means the full thumb image can be touched through a documented prop without breaking existing slider interactions; the issue names no files or tests, so identify the relevant coverage before changing it.

Written by the indexing model from the issue text.