file extension for schema documents

[d-v-b]

The schema documents are named with a .schema file extension. Because the schema documents are JSON schema, (which is JSON) .json is probably the right file extension to use instead. This change would make these documents much easier to read / edit, e.g. on github or in IDEs that provide json syntax highlighting. If it's important to designate these documents as JSON schema, we could do <name>.schema.json.

Is there a compelling reason for .schema as an extension, or can we move to schema.json?