Throw a useful error when tailwindcss is used as a PostCSS plugin

[philipp-spiess]

While upgrading a project to Tailwind CSS v4, I forgot to remove the tailwindcss import from the PostCSS config. As a result of this, I was greeted with the following message:

node:internal/process/promises:289
            triggerUncaughtException(err, true /* fromPromise */);
            ^

[Failed to load PostCSS config: Failed to load PostCSS config (searchPath: /Users/philipp/dev/project): [TypeError] Invalid PostCSS Plugin found at: plugins[0]

(@/Users/philipp/dev/project/postcss.config.js)
TypeError: Invalid PostCSS Plugin found at: plugins[0]

I don't think this was particularly helpful, so I’m proposing we add a default function export to the tailwindcss package so when it's used inside PostCSS, we can control the error message. So I changed it to something along these lines:

It looks like you're trying to use the \`tailwindcss\` package as a PostCSS plugin. This is no longer possible since Tailwind CSS v4.

If you want to continue to use Tailwind CSS with PostCSS, please install \`@tailwindcss/postcss\` and change your PostCSS config file.
    at w (/Users/philipp/dev/project/node_modules/tailwindcss/node_modules/tailwindcss/dist/lib.js:1:21233)
    at Object.<anonymous> (/Users/philipp/dev/project/node_modules/tailwindcss/postcss.config.cjs:3:13)
    at Module._compile (node:internal/modules/cjs/loader:1358:14)
    at Module._extensions..js (node:internal/modules/cjs/loader:1416:10)
    at Module.load (node:internal/modules/cjs/loader:1208:32)
    at Module._load (node:internal/modules/cjs/loader:1024:12)
    at cjsLoader (node:internal/modules/esm/translators:348:17)
    at ModuleWrap.<anonymous> (node:internal/modules/esm/translators:297:7)
    at ModuleJob.run (node:internal/modules/esm/module_job:222:25)
    at async ModuleLoader.import (node:internal/modules/esm/loader:316:24)

This is also a good place to link to the migration guides once we have them 🙂