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/HeapRoot.h

@@ -15,6 +15,7 @@ struct HeapRoot {
         HeapFunctionCapturedPointer,
         Handle,
         MarkedVector,
+        ConservativeVector,
         RegisterPointer,
         SafeFunction,
         StackPointer,