Replace ISelectionPosition API with IBufferRange

This will allow us to have generic interfaces to represent a buffer range vs a viewport range

```ts
interface IBufferRange {
  start: IBufferCellPosition;
  end: IBufferCellPosition;
}
interface IBufferPosition {
  x: number;
  y: number;
}
```

More context: https://github.com/xtermjs/xterm.js/pull/2470