LibJS: Add Handle<T>, a strong C++ handle for keeping GC objects alive

This is pretty heavy and unoptimized, but it will do the trick for now.
Basically, Heap now has a HashTable<HandleImpl*> and you can call
JS::make_handle(T*) to construct a Handle<T> that guarantees that the
pointee will always survive GC until the Handle<T> is destroyed.

Libraries/LibJS/Heap/Handle.cpp

@@ -0,0 +1,18 @@
+#include <LibJS/Heap/Handle.h>
+#include <LibJS/Heap/Heap.h>
+#include <LibJS/Runtime/Cell.h>
+
+namespace JS {
+
+HandleImpl::HandleImpl(Cell* cell)
+    : m_cell(cell)
+{
+    m_cell->heap().did_create_handle({}, *this);
+}
+
+HandleImpl::~HandleImpl()
+{
+    m_cell->heap().did_destroy_handle({}, *this);
+}
+
+}