fix(config): Add .mjs as an extension for Webpack to resolve

[nwalters512]

#107 dropped .mjs from the list of extensions Webpack will resolve. This is causing problems for us - graphql is in the import path of some of our components, and graphql/graphql-js#1272 is biting us. This cannot be fixed in userspace via webapckConfig in playroom.config.js, since the .mjs extension needs to appear before the .js extension, but the user-provided resolve.extensions is concatenated to the end of Playroom's base Webpack config.

An alternative solution would be to customize the way that webpack-merge merges resolve.extensions so that user-provided extensions always appear first. I'd be happy to pivot this PR to that if that's what the maintainers would prefer.

[maraisr]

mjs actually require a bit more than just resolution. You'd need to couple this with a webpack module rule:

config.module.rules.push({
	test: /\.mjs$/,
	include: /node_modules/,
	type: 'javascript/auto',
});

both of this you could just do in consume land with the webpack override.

[nwalters512]

Ah, interesting! Thanks for the tip. Before making this PR, I had tested locally by editing Playroom files in node_modules, and the build succeeded just with the addition of the .mjs extension. When I tested with Webpack config overrides, I didn't add the module rule. After adding that, things seem to work. Here's a simplified version of what my Playroom config looks like now:

// playroom.config.js
module.exports = {
  webpackConfig: () => ({
    resolve: {
      extensions: ['.mjs'],
    },
    module: {
      rules: [
        {
          test: /\.mjs$/,
          include: /node_modules/,
          type: 'javascript/auto',
        },
      ],
    },
  }),
};

Would be happy to add the module rule to this PR so that things "just work" for other folks.

[maraisr]

It all stems from Webpack not handling mjs properly. Or properly, who knows. The extension is fractured and nobody really knows! But this plugin resolved all:
https://github.com/lukeed/webpack-modules