Generate JSON Schema for all catalog item types

[staffordsmith83]

A tool to generate a json schema file for each catalog item type at build time.
This will allow us to have intellisense when editing json catalog items manually and avoid time consuming errors!

We would generate a json schema file from traits, similar to how generateDocs.ts works.
It would be good to include a script in terriajs package.json that will generate a folder of these json schema files.

Question: are our traits defined specifically enough so that we can generate a comprehensive json schema for each catalog item type? Do we define the possible values for each trait so that we can include validation patterns in the json schema? Should we define our traits even more strictly to facilitate this?