V2 : fix json import

[z3dev]

These changes fix issues when drag-n-drop of exported JSON files, i.e. designs saved as JSON. There were issues with the handling of single files, as well as a missing deserializer for JSON files.

UNDERNEATH... I've been wanting to refactor how the deserializers were registered in the IO module. The following changes have been made:

• create a list of deserializers by extention (new deserializers.js file)
• correct the index of IO module to include the deserializers
• change core transformSources to use the list of deserializers, simplify redundant verbose code
• change core registerExtensions to use the list of deserializers, simplify redundant verbose code
• change cli generateOutputData to use the list of deserializers

Now, there's just one list which can change, adding or removing a deserializer for an external format.

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• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• Does your submission pass tests?

[SimonClark]

Yes! And if there is a time to do this, it's now.

[SimonClark]

👍