Fix InsertRule to use the right CssRuleTypes (#32125)

`CSSRule::Type()` returns an u16 for CSSOM. `InsertRule()` was
incorrectly using this to create a `CssRuleTypes`.

Instead of `CssRuleTypes::from_bits(rule_type)`, it should be something
like `CssRuleTypes::from_bits(1 << rule_type)`.

However, that would only work when `Type()` provides an actual value,
which per https://drafts.csswg.org/cssom/#dom-cssrule-type only happens
for old rule types. New rule types just return 0.

Therefore, this patch changes the signature of `SpecificCSSRule::ty()`
to return the actual `CssRuleType`, and then `CSSRule::Type()` can zero
it out when necessary.

The fix is only relevant for CSS Nesting, which is currently disabled on
Servo, so no test is necessary.

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relevant for CSS Nesting, which is currently disabled

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components/script/dom/csskeyframerule.rs

@@ -6,6 +6,7 @@ use dom_struct::dom_struct;
 use servo_arc::Arc;
 use style::shared_lock::{Locked, ToCssWithGuard};
 use style::stylesheets::keyframes_rule::Keyframe;
+use style::stylesheets::CssRuleType;
 
 use crate::dom::bindings::codegen::Bindings::CSSKeyframeRuleBinding::CSSKeyframeRuleMethods;
 use crate::dom::bindings::inheritance::Castable;
@@ -73,9 +74,8 @@ impl CSSKeyframeRuleMethods for CSSKeyframeRule {
 }
 
 impl SpecificCSSRule for CSSKeyframeRule {
-    fn ty(&self) -> u16 {
-        use crate::dom::bindings::codegen::Bindings::CSSRuleBinding::CSSRuleConstants;
-        CSSRuleConstants::KEYFRAME_RULE
+    fn ty(&self) -> CssRuleType {
+        CssRuleType::Keyframe
     }
 
     fn get_css(&self) -> DOMString {