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/Heap/ConservativeVector.cpp

@@ -0,0 +1,23 @@
+/*
+ * Copyright (c) 2024, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibJS/Heap/ConservativeVector.h>
+#include <LibJS/Heap/Heap.h>
+
+namespace JS {
+
+ConservativeVectorBase::ConservativeVectorBase(Heap& heap)
+    : m_heap(&heap)
+{
+    m_heap->did_create_conservative_vector({}, *this);
+}
+
+ConservativeVectorBase::~ConservativeVectorBase()
+{
+    m_heap->did_destroy_conservative_vector({}, *this);
+}
+
+}