LibWeb: Use invalidation sets for :has() invalidation

Prior to this change, we invalidated all elements in the document if it
used any selectors with :has(). This change aims to improve that by
applying a combination of techniques:
- Collect metadata for each element if it was matched against a selector
  with :has() in the subject position. This is needed to invalidate all
  elements that could be affected by selectors like `div:has(.a:empty)`
  because they are not covered by the invalidation sets.
- Use invalidation sets to invalidate elements that are affected by
  selectors with :has() in a non-subject position.

Selectors like `.a:has(.b) + .c` still cause whole-document invalidation
because invalidation sets cover only descendants, not siblings. As a
result, there is no performance improvement on github.com due to this
limitation. However, youtube.com and discord.com benefit from this
change.

Libraries/LibWeb/CSS/SelectorEngine.h

@@ -18,6 +18,8 @@ enum class SelectorKind {
 
 struct MatchContext {
     GC::Ptr<CSS::CSSStyleSheet const> style_sheet_for_rule {};
+    GC::Ptr<DOM::Element const> subject {};
+    bool collect_per_element_selector_involvement_metadata { false };
     bool did_match_any_hover_rules { false };
 };