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;
    };

Meta/gn/secondary/Userland/Libraries/LibJS/BUILD.gn

@@ -44,6 +44,7 @@ shared_library("LibJS") {
     "Heap/BlockAllocator.cpp",
     "Heap/Cell.cpp",
     "Heap/CellAllocator.cpp",
+    "Heap/ConservativeVector.cpp",
     "Heap/Handle.cpp",
     "Heap/Heap.cpp",
     "Heap/HeapBlock.cpp",