Update Tailwind setup. Show extensions to install

[Tobbe]

I recently setup a new RW project with Tailwind and noticed a few improvements I thought could be made

First I wanted our setup to as closely as possible follow the latest instructions on https://tailwindcss.com/docs/installation/using-postcss
Mainly that meant to use @tailwind base; etc instead of @import ...

Unfortunately (?) I couldn't use their postcss config syntax, because they're using Next-specific syntax
tailwindlabs/tailwindcss.com#870 TW PR that updated their docs
vercel/next.js#10117 Next issue about the syntax with some good info in the comments, including motivation on why they use this non-standard syntax

Secondly I got syntax warnings in my css files after setting up Tailwind. After some googling I fixed that by installing the VS Code Tailwind intellisense extension. Looking at our code I then realized we do add that extension to the list of recommended extensions in the user's project. But we never actually tell the user to install it. I should have looked at all the files the setup command modified, but even if I had, I probably wouldn't have realized that was one of the plugins that we recommend, because the plugin ID doesn't match the name of the plugin (the ID is bradlc.vscode-tailwindcss)

This is the first warning I spotted (Unknown at rule @tailwind css(unknownAtRules))

This is what it looks like when running the setup command after my changes:

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@jtoar

• I selected the v6 milestone, but this totally doesn't have to go out in an RC. It can wait for v6.1 or whatever. It's non-breaking, but you decide when you want to release it.
• For the upgrade guide we could suggest people switch to the declaration syntax instead of imports. But the imports still seem to work, so we could also just skip it.

[jtoar]

Tried it locally and works, those extension messages are nice. Thanks @Tobbe!