This is the CSSOM interface that represents a nested declarations rule.
https://drafts.csswg.org/css-nesting/#the-cssnestrule
Testing: `/_mozilla/mozilla/interfaces.https.html`. And once
`CSSStyleRule` becomes a `CSSGroupingRule` subclass, this will be
further covered by `/css/css-nestting/`.
This is part of #36245
Signed-off-by: Oriol Brufau <obrufau@igalia.com>
