add script to generate a diagram of JDL grammar

[clementdessoude]

Hi @MathieuAA!

I just created this PR to show you a chevrotain tool that I found really useful in the Java parser we developed: it generates an HTML diagram of the JDL grammar. I think it could be useful when debugging some tricky bugs

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[clementdessoude]

I didn't see it was already done :)