LibJS: Replace StringOrSymbol::from_value with Value::to_property_key

This is a more specification compliant implementation of the
abstract operation 7.1.19 ToPropertyKey which should handle boxed
symbols correctly.

Userland/Libraries/LibJS/Forward.h

@@ -130,6 +130,7 @@ class ScopeNode;
 class ScopeObject;
 class Shape;
 class Statement;
+class StringOrSymbol;
 class Symbol;
 class Token;
 class VM;