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.

Userland/test-js.cpp

@@ -43,6 +43,8 @@
 
 #define TOP_LEVEL_TEST_NAME "__$$TOP_LEVEL$$__"
 
+RefPtr<JS::VM> vm;
+
 static bool collect_on_every_allocation = false;
 static String currently_running_test;
 
@@ -273,7 +275,10 @@ JSFileResult TestRunner::run_file_test(const String& test_path)
     currently_running_test = test_path;
 
     double start_time = get_time_in_ms();
-    auto interpreter = JS::Interpreter::create<TestRunnerGlobalObject>();
+    auto interpreter = JS::Interpreter::create<TestRunnerGlobalObject>(*vm);
+
+    // FIXME: This is a hack while we're refactoring Interpreter/VM stuff.
+    JS::VM::InterpreterScope scope(*interpreter);
 
     interpreter->heap().set_should_collect_on_every_allocation(collect_on_every_allocation);
 
@@ -603,6 +608,8 @@ int main(int argc, char** argv)
         DebugLogStream::set_enabled(false);
     }
 
+    vm = JS::VM::create();
+
 #ifdef __serenity__
     TestRunner("/home/anon/js-tests", print_times).run();
 #else
@@ -614,5 +621,7 @@ int main(int argc, char** argv)
     TestRunner(String::format("%s/Libraries/LibJS/Tests", serenity_root), print_times).run();
 #endif
 
+    vm = nullptr;
+
     return 0;
 }