Allow the dependency graph to be exported as JSON

[jjrscott]

Being able to export the source graph to other tools is extremely useful as it allows for usage outside the scope of the project without creating or requiring a public API (or the user having to create a swift wrapper just to get the information they need).

For example, with this change, it is possible to generate nice dependency charts such as described here: https://github.com/peripheryapp/periphery/discussions/758.