Adding notice of monorepo migration (#263)

* Using new package

* Using the new parser

* Simplifying logic given how new parser works

* Reworking the transforming logic to use the new parser

* Whitespaces changes

* Adding warning to README

README.md

@@ -1,3 +1,6 @@
+<div align="center">⚠️ PostCSS Color Custom Properties was moved to <a href="https://github.com/csstools/postcss-plugins/tree/main/plugins/postcss-custom-properties">@csstools/postcss-plugins</a>. ⚠️ <br>
+<a href="https://github.com/csstools/postcss-plugins/discussions/75">Read the announcement</a></div>
+
 # PostCSS Custom Properties [<img src="https://postcss.github.io/postcss/logo.svg" alt="PostCSS" width="90" height="90" align="right">][postcss]
 
 [![NPM Version][npm-img]][npm-url]