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/Runtime/GlobalObject.cpp

@@ -26,6 +26,7 @@
  */
 
 #include <AK/LogStream.h>
+#include <LibJS/Heap/DeferGC.h>
 #include <LibJS/Interpreter.h>
 #include <LibJS/Runtime/ArrayConstructor.h>
 #include <LibJS/Runtime/ArrayIteratorPrototype.h>
@@ -87,13 +88,12 @@ void GlobalObject::initialize()
     JS_ENUMERATE_BUILTIN_TYPES
 #undef __JS_ENUMERATE
 
-#define __JS_ENUMERATE(ClassName, snake_name)                                    \
-    if (!m_##snake_name##_prototype)                                             \
+#define __JS_ENUMERATE(ClassName, snake_name) \
+    if (!m_##snake_name##_prototype)          \
         m_##snake_name##_prototype = heap().allocate<ClassName##Prototype>(*this, *this);
     JS_ENUMERATE_ITERATOR_PROTOTYPES
 #undef __JS_ENUMERATE
 
-
     u8 attr = Attribute::Writable | Attribute::Configurable;
     define_native_function("gc", gc, 0, attr);
     define_native_function("isNaN", is_nan, 1, attr);