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/Runtime/Value.cpp

@@ -917,7 +917,7 @@ ThrowCompletionOr<PropertyKey> Value::to_property_key(VM& vm) const
     // 2. If key is a Symbol, then
     if (key.is_symbol()) {
         // a. Return key.
-        return &key.as_symbol();
+        return key.as_symbol();
     }
 
     // 3. Return ! ToString(key).