Vite: Add option to disable module-graph based scanning and turn it on for Astro

[philipp-spiess]

This PR adds an option to the Vite plugin to control wether or not the module-graph based dependency scanner is used.

The main motivation for doing this is a number of SSR specific build issues where differences in the dependency graphs for SSR and client builds would cause unexpected production builds. Some specific examples:

• Inconsistent CSS assets between client and SSR builds with @tailwindcss/vite #16389
• All the stylesheets CSS file loading twice on build #16252
• Astro client: directive will cause tailwindcss to be invalid #15794

The new option has a way to force module-graph and file-system based candidate scanning but will default to an automatic mode that will detect Astro and change the default behavior there to immediately fix the most prominent issue (#15794).

It is configured like this:

import { defineConfig } from 'vite'
import tailwindcss from '@tailwindcss/vite'

export default defineConfig({
  plugins: [tailwindcss({ scanner: 'file-system' })],
})

Test plan

• I updated the Astro integration test to include an example using the client:only directive that was able to reproduce the issue Astro client: directive will cause tailwindcss to be invalid #15794
• I also added an example that was able to reproduce the issue with React Router as outlined in All the stylesheets CSS file loading twice on build #16252
• The existing SSR tests were also able to reproduce the issues form Inconsistent CSS assets between client and SSR builds with @tailwindcss/vite #16389 by having both entry points use different class names.

I ensured that the new option turns-on automatically for Astro and that enabling it manually fixes the other cases.

[justingolden21]

If true that this would allow tailwind 4 to once again purge unused utility classes, then that would be an ENORMOUS win for developers and the web as a whole!

Thank you for working on this!

[philipp-spiess]

We're going with an option-less approach for now since it's less confusing and won't require debugging if things go wrong: #16631