Request: Add drag indicator as a separate component

[walmink]

Just started exploring this component and really liking it so far!

There is nice little drag indicator in the default header that changes shape depending on dragging direction. Right now it's hard-coded in the SheetHeader component, but I can imagine many people wanting to add their own header but still use that drag indicator.

It would be cool if either:

1. The drag indicator is its own component that can be plugged into a custom sheet header
2. Have the option to keep the indicator in place, even if adding a custom header