LibJS: Add source location for Handle nodes in GC graph dumper output

With this change JS::Handle root nodes will contain source location
where they were constructed like:
```
    "94675029575744": {
        "root": "Handle activate_event_handler \
           serenity/Userland/Libraries/LibWeb/DOM/EventTarget.cpp:564",
        "class_name": "HTMLButtonElement",
        "edges": [
            "94675025955904",
            "94675026899520",
            "94675030831168",
```

Userland/Libraries/LibJS/Heap/HeapRoot.h

@@ -0,0 +1,28 @@
+/*
+ * Copyright (c) 2023, Aliaksandr Kalenik <kalenik.aliaksandr@gmail.com>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/SourceLocation.h>
+
+namespace JS {
+
+struct HeapRoot {
+    enum class Type {
+        HeapFunctionCapturedPointer,
+        Handle,
+        MarkedVector,
+        RegisterPointer,
+        SafeFunction,
+        StackPointer,
+        VM,
+    };
+
+    Type type;
+    SourceLocation const* location { nullptr };
+};
+
+}