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/Forward.h

@@ -33,6 +33,7 @@ class Argument;
 class Cell;
 class Expression;
 class Function;
+class HandleImpl;
 class Heap;
 class HeapBlock;
 class Interpreter;
@@ -42,4 +43,7 @@ class ScopeNode;
 class Value;
 enum class DeclarationType;
 
+template<class T>
+class Handle;
+
 }