LibJS: Add JS_CELL macro and use it in all JS::Cell subclasses

This is similar to what we already had with JS_OBJECT (and also
JS_ENVIRONMENT) but sits at the top of the Cell inheritance hierarchy.

Userland/Libraries/LibJS/Heap/Cell.h

@@ -14,6 +14,14 @@
 
 namespace JS {
 
+#define JS_CELL(class_, base_class)                \
+public:                                            \
+    using Base = base_class;                       \
+    virtual StringView class_name() const override \
+    {                                              \
+        return #class_##sv;                        \
+    }
+
 class Cell {
     AK_MAKE_NONCOPYABLE(Cell);
     AK_MAKE_NONMOVABLE(Cell);