JSON Schema validation for exercises

[lgraziani2712]

Códimo uses JSON for the metadata of the exercises. Right now we have nothing to ease the validation of the schema.

Things to take into account:

• codimo-core needs to validate the core schema allowing custom definitions.
• Each activity could add custom properties to that schema just for itself.

This will allow better error detection in the instantiation phase of the activity without the components to have written each of the rules inside of them.

[lgraziani2712]

This https://www.npmjs.com/package/validated is the best package I could find related to this issue.