Create the @parcel/transform-postcss package in packages/transforms/postcss. This package is a Parcel 2 transform plugin for PostCSS. It will be based on the postcss transform inside CSSAsset from Parcel 1, separated into its own package.

The @parcel/transform-postcss package is responsible for the following things:

• Loading .postcssrc configs
• Parsing an AST if needed
• Transforming with PostCSS
• Generating a string if needed