LibJS: Add initial support for creating PrimitiveStrings with AK::String

This will temporarily bloat the size of PrimitiveString as LibJS is
transitioned to use String throughout, but will make doing so piecemeal
much easier.

Userland/Libraries/LibJS/Heap/Heap.cpp

@@ -57,6 +57,7 @@ Heap::Heap(VM& vm)
 
 Heap::~Heap()
 {
+    vm().string_cache().clear();
     vm().deprecated_string_cache().clear();
     collect_garbage(CollectionType::CollectEverything);
 }