different approach to generating schema

[mixmix]

This approach has advantages:

• the index.js isn't performing a surprise fs.writeFile
• the schema are generated with only the definitions they need
• there is a simple script for updating the schema.json
• the schema.js is no longer a fragment that could accidentally be used by itself, and being JS it supports comments
• what we export in JS is EXACTLY the same as what other languages would import from schema.json