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/Bytecode/Interpreter.cpp

@@ -1921,7 +1921,7 @@ static void dump_object(Object& o, HashTable<Object const*>& seen, int indent =
     seen.set(&o);
     for (auto& it : o.shape().property_table()) {
         auto value = o.get_direct(it.value.offset);
-        dbgln("{}  {} -> {}", String::repeated(' ', indent).release_value(), it.key.to_display_string(), value);
+        dbgln("{}  {} -> {}", String::repeated(' ', indent).release_value(), it.key.to_string(), value);
         if (value.is_object()) {
             dump_object(value.as_object(), seen, indent + 2);
         }