feat(nodes): Resizable nodes

There's some strange behaviour when nodes are resized: if the `` component re-mounts, the saved node sizes aren't restored. This may be related to how our nodes are all flex containers with relative sizes. It also affects the `reactflow` `` component.

Also, we probably want nodes to have dynamic minimum sizes - you shouldn't be able to reduce a node's size to the point where it overflows. It's not clear how to accomplish this.