Improve support for Tailwind CSS v4

[flavorjones]

Please do not ask for advice about v4 here.

Please go to TailwindCSS v4 - upgrade experience report · Discussion #450 for help, questions, and discussion.

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This issue is intended to drive concrete planning for better v4 support.

The state of the gem at the moment:

• tailwind css v4 was released 2025-01-23, and tailwindcss-ruby also released 4.0.0 the same day
• there is an upgrade process that existing applications should go through
  • we should tell people to stick on 3.x until they're ready to upgrade
  • we should document some steps to help people do this
    • doc: add upgrade notes to the README #467
  • if possible, we should automate the upgrade process with a generator/task
    • Introduce a 'tailwindcss:upgrade' task #466
• new rails applications don't seem to work properly
  • Installation should assume Tailwind CSS v4 #463

What else do we need to do?

Notably, I say "we" because I'm asking for help with this. I haven't upgraded a full app to v4 myself and so people who have should feel obligated to help get the gem into shape. Thanks in advance.

[flavorjones]

Closing. Shipped a release candidate: Release v4.0.0.rc1 / 2025-01-23 · rails/tailwindcss-rails