add JSON Schema

[StefanFl]

This JSON Schema can be used to check the syntax of a CLE document. Some remarks:

• The $id https://cle.example.com/schema/cle-1.0.0.schema.json fits to the specification document, but the hostname looks a bit random.
• Rules like the event id must be in descending order or the supportId must reference to a support policy defined in the definitions section can not be checked with the JSON Schema.
• The Schema has been tested with some CLE documents written by myself, but some independing testing would be great.
• The Schema is a quite lengthy, but it was the only way I could make it work as best as possible.

[ljharb]

there's lots of changes still needed here, but it's a great first start!

[ljharb]

yeah, i think this needs to be an actual URL on tc54.org. @stevespringett, can we get this (and the spec) deployed there?

[StefanFl]

Thanks @ljharb for the initial review. I have made the definitions more clear. There are still property definitions without types. These are needed because of how "additionalProperties": false behaves.

[noqcks]

@stevespringett to get the schema domain at tc54.org