fix(css parser): handle nested atRule

[noreiller]

Issue:
When using the CSS experiments and using @media or @supports in CSS code, the nested class names are not replaced with their unique identifier.

Changes:
This PR will handle @media and @supports atRule identifiers and enable the replacements of the nested rules names with the unique ids.

CSS input code:

.main {
	font-size: var(--large);
	color: darkblue;
}
@media (min-width: 1024px) {
	.main {
		color: green;
	}
}
@supports (display: grid) {
	.main {
		display: grid
	}
}

The compiled CSS code with the issue:
You can see that only the top-level class name is replaced.

.app-491-D {
	font-size: var(--large);
	color: darkblue;
}
@media (min-width: 1024px) {
	.main {
		color: green;
	}
}
@supports (display: grid) {
	.main {
		display: grid
	}
}

The compiled CSS code thanks to the fix:
You can see that all the class names are replaced.

.app-491-D {
	font-size: var(--large);
	color: darkblue;
}
@media (min-width: 1024px) {
	.app-491-D {
		color: green;
	}
}
@supports (display: grid) {
	.app-491-D {
		display: grid
	}
}

[linux-foundation-easycla]

• ❌ - login: @noreiller / name: Aurélien MANCA . The commit (7b71e81) is not authorized under a signed CLA. Please click here to be authorized. For further assistance with EasyCLA, please submit a support request ticket.

[webpack-bot]

For maintainers only:

• This needs to be documented (issue in webpack/webpack.js.org will be filed when merged)
• This needs to be backported to webpack 4 (issue will be created when merged)

[vankop]

please accept cla

[noreiller]

please accept cla

@vankop I contacted my company to accept CLA as a Corporate Contributor. I hope this will be done next week.

[sokra]

This also needs a test case. The example is not enough to test the behavior

[webpack-bot]

@noreiller Thanks for your update.

I labeled the Pull Request so reviewers will review it again.

@sokra Please review the new changes.

[noreiller]

This also needs a test case. The example is not enough to test the behavior

Hello @sokra , thanks for your review, I added some config cases tests.

[noreiller]

Due to technical issues between the Linux foundation and my company to sign the CLA, I close this PR and created a new one on my personal github.