Migrate simple PostCSS setup

[philipp-spiess]

This PR attempts to detect simple postcss setups: These are setups that do not load dynamic modules and are based off the examples we are recommending in our docs. We detect wether a config is appropriate by having it use the object plugin config and by not requiring any other modules:

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

When we find such a config file, we will go over it line-by-line and attempt to:

• Upgrade tailwindcss: to '@tailwindcss/postcss':
• Remove autoprefixer if used

We then attempt to install and remove the respective npm packages based on the package manger we detect.

And since we now have logic to upgrade packages, this also makes sure to install tailwindcss@next at the end of a sucessful migration.

[RobinMalfait]

We have some failing tests 🤔

[philipp-spiess]

Some comments from my call with @RobinMalfait that I will fix up 👍

[thecrypticace]

Just a couple things but otherwise looks good!

fixed