Use PostCSS loader if a PostCSS config file is present

[bjackson]

This will allow users to use PostCSS without modifying their webpack configs.

To enable the postcss-loader, add a postcss.config.js file to web/config.

As an example, you can use this config file to enable autoprefixer. (Install yarn workspace web add autoprefixer to use this).

module.exports = {
    plugins: {
        'autoprefixer': {},
    }
}

[thedavidprice]

Thanks for this @bjackson!

Question: does it look like we need to update this doc based on the changes in this PR?
https://redwoodjs.com/docs/webpack-configuration#supported-extensions-and-loaders

If so, please loop in @jtoar and me and we'll make sure to update it.

[bjackson]

@thedavidprice Yes, we should update the docs to include adding a config to web/config/postcss.config.js and installing postcss-loader, instead of altering the postcss-loader options.

[jtoar]

Thanks for this @bjackson, doing docs on this now in https://github.com/redwoodjs/redwoodjs.com/pull/219.