Prevent data duplication with serializeGraph when using object nodes

[JesusTheHun]

This PR adds a new optional argument to the serializeGraph function.
Similar to the identity function of deserializeGraph, the new argument is a function that returns a value that uniquely identify a node.

This value is then used when serializing links, leading to a more compact serialization. The serialization of nodes is not affected.

const nodeA = { id: 1, title: 'a' };
const nodeB = { id: 2, title: 'b' };

const graph = new Graph<{ id: number; title: string }>();
graph.addEdge(nodeA, nodeB);

const serialized = serializeGraph(graph);
// { links: [{ source: { id: 1, title: 'a' }, target: { id: 2, title: 'b' } }]}

const serializedWithIdentity = serializeGraph(graph, n => n.id);
// { links: [{ source: 1, target: 2 }]}

[curran]

LGTM! Thank you.

[curran]

Released in https://github.com/datavis-tech/graph-data-structure/releases/tag/v4.3.0