enhancement/issue 1312 ESM-only PostCSS configuration file

[thescientist13]

Related Issue

resolves #1312

Summary of Changes

1. Only check for an ESM based configuration file
2. Update PostCSS related dependencies
3. Update plugin README

Breaking change will require renaming postcss.config.mjs -> postcss.config.js

Documentation

1. Create issue to update PostCSS website docs - update PostCSS docs to reflect ESM-only configuration www.greenwoodjs.dev#145

TODO

1. Sanity check, since in retrospect, seems like we never needed this, since we programmatically loaded the configuration file ourselves? - [DEMO] ESM-only PostCSS config AnalogStudiosRI/www.blissri.com#54
   • works just fine as is lol 😅
2. restore custom configuration docs - https://github.com/ProjectEvergreen/greenwood/pull/1332/files#r1864679805

[lschierer]

I think the difference is that https://github.com/postcss/postcss-load-config now supports a zillion different formats but your plugin documentation never got updated when it did, as it points at a really old issue :-)

[thescientist13]

Yeah, I think this was just an implementation error on our part since we don't actually rely on PostCSS to find the configuration file, we feed the options directly to PostCSS ourselves so in retrospect the .cjs variant was never needed probably 🤦‍♂️ 😅