LibJS+LibWeb: Replace StringOrSymbol usage with PropertyKey

- Avoids unnecessary conversions between StringOrSymbol and PropertyKey
  on the hot path of property access.
- Simplifies the code by removing StringOrSymbol and using PropertyKey
  directly. There was no reason to have a separate StringOrSymbol type
  representing the same data as PropertyKey, just with the index key
  stored as a string.

PropertyKey has been updated to use a tagged pointer instead of a
Variant, so it still occupies 8 bytes, same as StringOrSymbol.

12% improvement on JetStream/gcc-loops.cpp.js
12% improvement on MicroBench/object-assign.js
7% improvement on MicroBench/object-keys.js

Libraries/LibJS/AST.cpp

@@ -243,7 +243,7 @@ ThrowCompletionOr<ClassElement::ClassValue> ClassField::class_element_evaluation
             auto copy_initializer = m_initializer;
             auto name = property_key_or_private_name.visit(
                 [&](PropertyKey const& property_key) -> String {
-                    return property_key.is_number() ? property_key.to_string() : property_key.to_string_or_symbol().to_display_string();
+                    return property_key.to_string();
                 },
                 [&](PrivateName const& private_name) -> String {
                     return private_name.description.to_string();