Create JSON schema for piling configuration

[flekschas]

This issue is related to #46

Once we are able to import/export the piling configuration, we should define a JSON schema to describe the piling configurations. We can use this tool to get started, which turns a JSON object into a JSON schema: https://jsonschema.net/

The root $id should be https://piling.js.org/schema.json

Once we have the basics we need to manually define some other useful things like value ranges (which the tool from above can't extract automatically): https://json-schema.org/understanding-json-schema/reference/numeric.html