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/Makefile

@@ -1,5 +1,6 @@
 OBJS = \
     AST.o \
+    Heap/Handle.o \
     Heap/Heap.o \
     Heap/HeapBlock.o \
     Interpreter.o \