d.ts as JSON file

[duaneatat]

This adds a script that will generate a single types/types.json file which is easier to consume in a TypeScript VFS that is bundled by webpack than regular d.ts files. Once we've finished migrating to TypeScript, we can also publish the d.ts files (as well as point to them in the package.json).

Will hold off on merging this until #1016 is reviewed/merged.