Output the dependency graph of the current Catala module in JSON format

[denismerigoux]

This is a request by @EstelleHary : writing a compiler plugin that takes the dependency graphs (intra-scope and inter-scope) and output them as JSON for her to explore.

These are the data structures to serialize :

https://github.com/CatalaLang/catala/blob/master/compiler/desugared/dependency.mli

https://github.com/CatalaLang/catala/blob/master/compiler/scopelang/dependency.mli