LibJS+Clients: Add JS::VM object, separate Heap from Interpreter

Taking a big step towards a world of multiple global object, this patch
adds a new JS::VM object that houses the JS::Heap.

This means that the Heap moves out of Interpreter, and the same Heap
can now be used by multiple Interpreters, and can also outlive them.

The VM keeps a stack of Interpreter pointers. We push/pop on this
stack when entering/exiting execution with a given Interpreter.
This allows us to make this change without disturbing too much of
the existing code.

There is still a 1-to-1 relationship between Interpreter and the
global object. This will change in the future.

Ultimately, the goal here is to make Interpreter a transient object
that only needs to exist while you execute some code. Getting there
will take a lot more work though. :^)

Note that in LibWeb, the global JS::VM is called main_thread_vm(),
to distinguish it from future worker VM's.

Libraries/LibJS/Heap/Heap.cpp

@@ -47,8 +47,8 @@
 
 namespace JS {
 
-Heap::Heap(Interpreter& interpreter)
-    : m_interpreter(interpreter)
+Heap::Heap(VM& vm)
+    : m_vm(vm)
 {
 }
 
@@ -57,6 +57,11 @@ Heap::~Heap()
     collect_garbage(CollectionType::CollectEverything);
 }
 
+Interpreter& Heap::interpreter()
+{
+    return vm().interpreter();
+}
+
 Cell* Heap::allocate_cell(size_t size)
 {
     if (should_collect_on_every_allocation()) {
@@ -100,7 +105,8 @@ void Heap::collect_garbage(CollectionType collection_type, bool print_report)
 
 void Heap::gather_roots(HashTable<Cell*>& roots)
 {
-    m_interpreter.gather_roots({}, roots);
+    if (auto* interpreter = vm().interpreter_if_exists())
+        interpreter->gather_roots({}, roots);
 
     gather_conservative_roots(roots);