Use shared helpers for generated functions

[ascorbic]

Rather than creating each function with lots of duplicated files, make the wrapper into an export which is imported from a shared lib. So rather than creating a directory like this:

...we have just the entry point file next_index.js and the nextPage directory. The entry point would be something like:

const functionHandler= require("netlify-plugin-netlify/functions")
const nextPage = require('./nextPage')

module.exports = functionHandler({ nextPage })