Improve nesting detection

[RobinMalfait]

This PR improves the nesting detection.

Currently we only detected rules inside of rules, and if we did then we would trigger a PostCSS warning.

However, some people try to do something like this:

.namespace {
  @tailwind base;
  @tailwind components;
  @tailwind utilities;
}

While in theory this could work~ish, this is not something we support. The biggest reason is that base styles (and thus also the preflight/reset) would end up looking like this: .namespace html {} for example which doesn't make much sense.

This PR also detects @tailwind at rules, with a parent that is not of type root and shows a similar message:

Nested @tailwind rules detected, instead try to use a prefix: https://tailwindcss.com/docs/configuration#prefix or scope it using the important selector strategy https://tailwindcss.com/docs/configuration#selector-strategy

(@reinink can you double check the wording used in the message? Or do we keep it as is and improve all warning messages before v3 launch?)

[reinink]

Here are some ideas.

First, for nested Tailwind at-rules:

warn - Nested @tailwind rules were detected, but are not supported.
warn - Consider using a prefix to scope Tailwind's classes: https://tailwindcss.com/docs/configuration#prefix
warn - Alternatively, use the important selector strategy: https://tailwindcss.com/docs/configuration#selector-strategy

And for just plain nesting:

warn - Nested CSS was detected, but CSS nesting has not been configured correctly.
warn - Please enable a CSS nesting plugin *before* Tailwind in your configuration.
warn - See how here: https://tailwindcss.com/docs/using-with-preprocessors#nesting

[RobinMalfait]

Thanks @reinink, I'll merge this with the provided changes. If we need further cleanup in warning messages (which we probably will if we want to add links to documentation pages).