Integrate a JSON-Schema validator for the Moodle-Tools YAML Schema

Related to #11 (closed) and #24, once having agreed on the general schema for all question types, having a proper json-schema for both highlighting and autocompletion, the schema can be used to validate the yaml input file, instead of using the if-else error array in the validate() method for each type. In addition, not all the question types implement their own, some extend and inherit that method.

One potential library is jsonschema https://github.com/python-jsonschema/jsonschema, as mentioned in the JSON validate example at https://json-schema.org/overview/what-is-jsonschema