Auto merge of #16315 - jryans:at-page-stylo, r=xidorn

Stylo: @page support

Reviewed by upsuper in https://bugzilla.mozilla.org/show_bug.cgi?id=1345206.

- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors

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components/script/dom/cssrule.rs

@@ -83,6 +83,7 @@ impl CSSRule {
             StyleCssRule::Namespace(s) => Root::upcast(CSSNamespaceRule::new(window, parent_stylesheet, s)),
             StyleCssRule::Viewport(s) => Root::upcast(CSSViewportRule::new(window, parent_stylesheet, s)),
             StyleCssRule::Supports(s) => Root::upcast(CSSSupportsRule::new(window, parent_stylesheet, s)),
+            StyleCssRule::Page(_) => unreachable!(),
         }
     }