Migrate to Tailwind 4

[infomiho]

Tailwind CSS v4.0 is an all-new version of the framework optimized for performance and flexibility, with a reimagined configuration and customization experience, and taking full advantage of the latest advancements the web platform has to offer.

We should follow the upgrade guide and see how we want to deal with Tailwind in the future in Wasp.

Now we are installing a dependency if they have certain config files present. But we require them to have this dependency #2465 anyways (due to problems with how npm resolves peer deps) - so this begs the question, do we provide any value? Should we make Tailwind a userland thing?

In version 4, the setup is quite simple: install a Vite plugin, add it to your Vite config and include import "tailwind"; in a CSS file. Simpler than before.