Create example of using eslint to ensure Firefox compatibility

[kumar303]

Firefox supports most of ES6 but not all of it. For example, it doesn't support the import statement at the time of this writing.

Let's create an example of an extension that is configured with eslint to make sure that its code uses only JavaScript that is supported by Firefox. This example extension would not use a transpiler on the code.

I think there are some pre-configured lint rules to use, such as https://www.npmjs.com/package/eslint-plugin-mozilla

[kumar303]

Thanks again for this @iampeterbanjo ! This landed in #128