Expose firefox global require

This pull request adds a Cu.import style module for sharing one global require, this enables anyone to get require as:

 ```js
const { require } = Cu.import("resource://gre/modules/commonjs/toolkit/require.js", {})
```
This is not ideal, but would enable us to use `require` in frame scripts and elsewhere.

P.S. I was unable to include test as we first need to land this under `resource://gre/modules/commonjs/toolkit/require.js`