allow users to pass in their own resolve field

[koddsson]

react-relay-networkd-modern needs to read .mjs for some reason. And I would like to use it in my project. The README instructs me to add this to my webpack config but webpack-config-github doesn't read it:

resolve: {
 extensions: ['.mjs', '.js']
}

This patch allows users of this package to pass in a resolve value and will make sure resolve.mainFields includes the correct values in addition to whatever the user supplied.

[josh]

I'd be fine just adding support for mjs for out of the box support instead of opening up a resolve option. We could maybe even add a test for this case 😁

[koddsson]

Sounds good!

[koddsson]

Webpack 4 has .mjs in the list of extensions. I couldn't find documentation for webpack 3 so I can't reference that but I'm guessing extensions doesn't include .mjs since the tests I added fail with out the .mjs extension in the list.

[koddsson]

Once my relay-compiler PR is merged and we get out of this dependency hell I can merge make a test, merge this and make another PR to resolve .ts and .tsx files as well.

[koddsson]

Once my relay-compiler PR is merged and we get out of this dependency hell I can merge make a test, merge this and make another PR to resolve .ts and .tsx files as well.

Oh I had already made a test, I guess I'm merging this 😆

[josh]

Nice! Thanks for adding this.