Code editor for proto nodes

[Keavon]

We should add a Monaco code editor for the Rust code which provides the implementations for proto nodes, as well as surrounding proto node metadata. This would allow editing a proto node implementation or creating one's own proto nodes with new algorithms. This should be approachable once the prerequisites like #1300 are unblocked, but to move from an MVP to a more complete feature it'll require some design work for the UI as well.