Select Tool: Viewport box selection

[Keavon]

Extending upon #140, we need to drag a box around a region in the viewport and select all the layers inside (which would be selected if you went and selected every single pixel inside that rectangular region). We don't want to use the bounding box of each layer, because your box selection might include shapes which don't have geometry located in the rectangular region. This will possibly be tricky to solve and might need to use some of Kurbo's algorithms. It would be nice if we can do it purely in the frontend with JS though. Depends on #137.

Complexity: 3
Involves: Frontend (JS), Editor (Rust)

[Keavon]

Blocked presently on #100.