We bundle cssnano in our CLI and it's on an older version that does not
handle nested CSS correctly. This PR updates it to the latest version of
v6.x

Fixes #14092

…14140)

When you use a glob pattern in your `content` configuration that is too
broad, it could be that you are accidentally including files that you
didn't intend to include. E.g.: all of `node_modules`

This has been documented in the [Tailwind CSS
documentation](https://tailwindcss.com/docs/content-configuration#pattern-recommendations),
but it's still something that a lot of people run into.

This PR will try to detect those patterns and show a big warning to let
you know if you may have done something wrong.

We will show a warning if all of these conditions are true:

1. We detect `**` in the glob pattern
2. _and_ you didn't explicitly use `node_modules` in the glob pattern
3. _and_ we found files that include `node_modules` in the file path
4. _and_ no other globs exist that explicitly match the found file

With these rules in place, the DX has nice trade-offs:

1. Very simple projects (that don't even have a `node_modules` folder),
can simply use `./**/*` because while resolving actual files we won't
see files from `node_modules` and thus won't warn.
2. If you use `./src/**` and you do have a `node_modules`, then we also
won't complain (unless you have a `node_modules` folder in the `./src`
folder).
3. If you work with a 3rd party library that you want to make changes
to. Using an explicit match like `./node_modules/my-package/**/*` is
allowed because `node_modules` is explicitly mentioned.

Note: this only shows a warning, it does not stop the process entirely.
The warning will be show when the very first file in the `node_modules`
is detected.

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Co-authored-by: Adam Wathan <adam.wathan@gmail.com>