Hook up document scrollbars and make them interactable

[Keavon]

Scrollbars are visually implemented but they need to work based on user interaction, and need to be hooked up to the backend to display the current scroll status and make the interaction tell the backend about the scrolling. This includes dragging the bar, clicking in the track to the sides of the bar, and clicking the arrows at the ends of the bar. Some of this will be implemented in the PersistentScrollbar.vue component and some will be implemented in the Document.vue component. For an infinite canvas, the bar's width should be based on the bounding box of the artwork in the canvas (from the angle of the current rotation of the viewport, just like you'd see if you selected all the layers in the document and saw the Select Tool's selection bounding box), and it should be based on the artboard dimensions when the canvas is not an infinite canvas.