Machine readable nix why-depends

[DieracDelta]

Is your feature request related to a problem? Please describe.
I'd like to query nix why-depends and parse the output. The output is currently unstructured.

Describe the solution you'd like
Ideally a cli option for returning the output as JSON or another similar format would make this much easier.

Describe alternatives you've considered
N/A

Additional context
N/A

Priorities

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[roberth]

JSON would be our choice, as we already have some commands that support --json.

The architecture for this kind of thing is to have it produce typed C++ data that can be rendered either to JSON or to the terminal.
We also have guidelines for JSON output.