Build a graphviz format visualizer for the runtime

[jverkoey]

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We can currently generate graphviz output of the runtime with runtime.toGraphviz(), but we don't have an easy way to represent this information natively. The swift playground is simply posting the graphviz to a website and rendering it there, but ideally we'd have some form of native visualization.

This could be built in multiple phases:

Phase 1: Read-only

A static visualization of the runtime's connections as a graph.

Phase 2: Modifying reactive properties

Any reactive property detected in the graph should be tweakable.

Phase 3: Creation

Being able to add new connections to the runtime graph; e.g. constraints.