allow users to pass in their own resolve field

examples/mjs/index.js

@@ -0,0 +1,3 @@
+const subject = require('./lib')
+
+document.body.innerHTML = `<h1>Hello, ${subject}!</h1>`

examples/mjs/lib.mjs

@@ -0,0 +1 @@
+export default 'world'

examples/mjs/webpack.config.js

@@ -0,0 +1,5 @@
+/* @flow strict */
+
+const config = require('../..') // require('webpack-config-github')
+
+module.exports = config

webpack.config.js

@@ -287,6 +287,7 @@ module.exports = (env /*: string */ = 'development', options /*: Options */) =>
   }
 
   config.resolve = {
+    extensions: ['.mjs', '.js'],
     mainFields: ['style', 'browser', 'module', 'main']
   }