[v4][CLI] add the --postcss flag back

[GrimLink]

In Tailwind CLI v3, we leveraged custom PostCSS plugins to effectively manage external Tailwind imports within our Magento 2 theme. This enabled seamless integration and streamlined development. However, v4 removed this capability, creating a significant obstacle for our upgrade.

We relied on this feature to resolve imports from vendor directories outside the primary Tailwind source. While we've explored using Vite as an alternative for CSS compilation and file watching, it introduces several drawbacks. Firstly, it necessitates duplicating source paths, leading to maintenance overhead. Secondly, it fails to monitor changes in external vendor files, impacting our development efficiency.

Restoring the ability to extend the Tailwind CLI with custom actions, specifically regarding PostCSS plugins, would directly address our needs and remove the current blocker preventing our team from migrating to v4.

[wongjn]

Perhaps you could consider using the PostCSS CLI and integrating Tailwind as a PostCSS plugin?

[GrimLink]

I did, sadly the postcss-cli can't watch for files other then wat you want to compile, so no phtml files.