Implement CSSNestedDeclarations (#36248)

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>

components/script/dom/mod.rs

@@ -271,6 +271,7 @@ pub(crate) mod csslayerblockrule;
 pub(crate) mod csslayerstatementrule;
 pub(crate) mod cssmediarule;
 pub(crate) mod cssnamespacerule;
+pub(crate) mod cssnesteddeclarations;
 pub(crate) mod cssrule;
 pub(crate) mod cssrulelist;
 pub(crate) mod cssstyledeclaration;