zapier convert: Use Prettier to format generated code

[eliangcs]

We use these templates to generate code for converted apps. The generated code is not pretty and it's not easy to manually format code in the templates. This PR adds Prettier into the conversion process so all the rendered .json and .js files are prettified with JSON.stringify and Prettier respectively before they get written to the filesystem.

[xavdid]

love it! I really enjoy prettier. I'd even go a step further and use it and a pre-commit hook on all the code in this and core. I know the front end team uses it. I disagree with their config a bit, but like the tool a lot.