fix: allow custom config file for postcss again

Created by: kitsunekyo

this resolves #11920 this was possible before, but was broken with 5614c87b

before commit 5614c87b it was possible to use a custom postcss.config.js file to enable/customize postcss plugins and more. this is required for tailwind users that want to use common features like postcss-nesting or postcss-import. https://tailwindcss.com/docs/using-with-preprocessors#build-time-imports

here is the docu to the webpack config option that should not be set to false: https://webpack.js.org/loaders/postcss-loader/#config

Reproduction Steps

see CONTRIBUTING.md to test locally. in the newly generated app (using the updated react-scripts config of course) apply these changes:

npm i postcss-import tailwindcss

<!-- append to src/App.tsx, in the <header> section -->
<input
    type="submit"
    className="btn btn-primary mt-8 px-5 py-3"
    value="Click me"
 />

/* add to top of src/index.css */
@import "tailwindcss/base";
@import "tailwindcss/components";
@import "./button.css";

@import "tailwindcss/utilities";

/* src/button.css */
.btn {
  @apply cursor-pointer inline-block rounded-md text-base tracking-wide font-medium px-4 py-2;
}
.btn-primary {
  @apply bg-indigo-500 text-white;
}
.btn-primary:hover {
  @apply bg-indigo-400;
}

// tailwind.config.js
module.exports = {
  content: ["./src/**/*.tsx", "./src/**/*.ts"],
  theme: {},
};

// postcss.config.js
module.exports = {
  plugins: [
    require("postcss-import"),
    require("tailwindcss"),
    require("autoprefixer"),
  ],
};

when running the app with npm start you should see a correctly styled tailwind button. without this change, the button styles are not correctly applied due to broken import order without postcss-import.

see my original issue on tailwind https://github.com/tailwindlabs/tailwindcss/discussions/7049