Small bug fix when calling generateElementRangeComponent

Avoid error being thrown when setting start/end of a range without specifying an offset.

Specifically, when generating a range CFI using full nodes, the epub library will throw an error because the native Range object's `setStart` and `setEnd` functions expect 2 arguments instead of just one. 

Replaced this code with `setStartBefore` and `setEndAfter` to create a valid range encompassing these two nodes.