LibJS: Add ConservativeVector<T>

This works very similarly to MarkedVector<T>, but instead of expecting
T to be Value or a GC-allocated pointer type, T can be anything.
Every pointer-sized value in the vector's storage will be checked during
conservative root scanning.

In other words, this allows you to put something like this in a
ConservativeVector<Foo> and it will be protected from GC:

    struct Foo {
        i64 number;
        Value some_value;
        GCPtr<Object> some_object;
    };

Userland/Libraries/LibJS/Forward.h

@@ -302,6 +302,9 @@ class ThrowCompletionOr;
 template<class T>
 class Handle;
 
+template<class T, size_t inline_capacity = 0>
+class ConservativeVector;
+
 template<class T, size_t inline_capacity = 0>
 class MarkedVector;