* Insert a new whitespace of a certain height after a line number.
* The whitespace has a "sticky" characteristic.
* Irrespective of edits above or below `afterLineNumber`, the whitespace will follow the initial line.
*
* @param afterLineNumber The conceptual position of this whitespace. The whitespace will follow this line as best as possible even when deleting/inserting lines above/below.
* @param heightInPx The height of the whitespace, in pixels.
* @return An id that can be used later to mutate or delete the whitespace