Generate visual representation of the index graph

With https://github.com/Shopify/index/pull/69 we're moving the implementation of the index to a graph structure:

• Nodes: names, definitions, URIs
• Edges: name -> definitions, definition -> name, uri -> definition

Vini provided a visualization of the structure:

Let's provide a way to automatically generate this kind of representation directly backed into the index.

So for example this:

$ index_cli path/to/files --dot graph.dot

Would directly generate the visual representation of the graph in DOT format. One can take inspiration from how we do it in Spoom's POSet: https://github.com/Shopify/spoom/blob/main/lib/spoom/poset.rb#L101-L129

Of course as we aim to index quite large codebases, we should find ways to make the output actually readable:

• Should we be able to filter the edges we want to show?
• Or the kind of nodes?
• Or to focus on a node and only show n-parents/children?
• Or is there a way to have an interactive map of the graph?
• Should we also have a --mermaid output?

Start with the index_cli entry point and the graph structure described in the issue, then review Spoom's POSet implementation for DOT generation. Define the supported filters, node and edge selection, and whether Mermaid or interactive output is in scope before implementing. Done means the index can generate a readable graph representation for a specified codebase.