LibJS: Keep track of PrimitiveStrings and share them

VM now has a string cache which tracks all live PrimitiveStrings and
reuses an existing one if possible. This drastically reduces the number
of GC-allocated strings in many real-word situations.

Userland/Libraries/LibJS/Heap/Heap.cpp

@@ -52,6 +52,7 @@ Heap::Heap(VM& vm)
 
 Heap::~Heap()
 {
+    vm().string_cache().clear();
     collect_garbage(CollectionType::CollectEverything);
 }