Bug 258755

Undefined in HashSet<CSSSelector::PseudoClassType>:
```
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior /Volumes/Work/WebKit/OpenSource/WebKitBuild/Release/usr/local/include/wtf/RefPtr.h:75:82 in 
/Volumes/Work/WebKit/OpenSource/WebKitBuild/Release/usr/local/include/wtf/HashTable.h:301:114: runtime error: load of value 4294967295, which is not a valid value for type 'const WebCore::CSSSelector::PseudoClassType'
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior /Volumes/Work/WebKit/OpenSource/WebKitBuild/Release/usr/local/include/wtf/HashTable.h:301:114 in 
/Volumes/Work/WebKit/OpenSource/WebKitBuild/Release/usr/local/include/wtf/HashFunctions.h:104:46: runtime error: load of value 4294967295, which is not a valid value for type 'WebCore::CSSSelector::PseudoClassType'
```

The issue is that CSSSelector is not a scoped enumeration but we use `StrongEnumHashTraits<CSSSelector::PseudoClassType>` for HashSets (in RuleFeature.h for example).
This means we end up using `std::underlying_type<CSSSelector::PseudoClassType>::max()` and `std::underlying_type<CSSSelector::PseudoClassType>::max() -1` as special HashMap values. Casting values outside the enum value range to an enumeration that is not scoped is undefined behavior.

To address the issue, I am converting CSSSelector::PseudoClassType to a scoped enumeration.