Replace ISelectionPosition API with IBufferRange

[Tyriar]

This will allow us to have generic interfaces to represent a buffer range vs a viewport range

interface IBufferRange {
  start: IBufferCellPosition;
  end: IBufferCellPosition;
}
interface IBufferPosition {
  x: number;
  y: number;
}

More context: #2470

[Tyriar]

Thought: this position is only valid immediately after it was generated, y would need to be a marker for any longer.

[jerch]

Thought: this position is only valid immediately after it was generated, y would need to be a marker for any longer.

Certainly a problem - could we get this done with markers? Not sure about their maintenance/update penalty, if there are a few held...

[Tyriar]

Should be fine if there aren't that many, marker have overhead of initializing an object and having their line updated on every scroll once scrollback is maxed out.