Boolean operations node

[Keavon]

The legacy boolean operations were unstable, and are no longer compatible with the migration of vector data to nodes (#1065) so they had to be removed. But they're very important in the artist's workflow, so re-adding them as a node is high priority.

In order of importance, this should:

• [ ] Either use web-sys to call Paper.js, or finish the attempted Paper.js Rust port in this branch
• [ ] Introduce a vector operations node operating on vector data in the graph
• [ ] Hook up the boolean buttons in the Select tool's options bar so they take the selected vector layers and combine them into one graph set up so they feed into the boolean operations node

Each of these would be suitable for a single or separate PRs depending on the author's approach.