LibJS: Add Handle specialisation for Value

This allows you to keep an arbitrary JS::Value alive without having to
hook visit_edges somewhere, e.g. by being a NativeFunction that
overrides visit_edges.

For example, this allows you to store JS::Handle<JS::Value> as the key
of a HashMap. This will be used to keep arbitrary Values alive in
the key of a temporary HashMap in Array.prototype.groupByToMap.

Co-authored-by: Ali Mohammad Pur <mpfard@serenityos.org>

Userland/Libraries/LibJS/Heap/Handle.h

@@ -12,6 +12,7 @@
 #include <AK/RefCounted.h>
 #include <AK/RefPtr.h>
 #include <LibJS/Forward.h>
+#include <LibJS/Runtime/Value.h>
 
 namespace JS {
 
@@ -77,4 +78,42 @@ inline Handle<T> make_handle(T& cell)
     return Handle<T>::create(&cell);
 }
 
+template<>
+class Handle<Value> {
+public:
+    Handle() = default;
+
+    static Handle create(Value value)
+    {
+        if (value.is_cell())
+            return Handle(value, &value.as_cell());
+        return Handle(value);
+    }
+
+    auto cell() { return m_handle.cell(); }
+    auto cell() const { return m_handle.cell(); }
+    auto value() const { return m_value; }
+    bool is_null() const { return m_handle.is_null(); }
+
+private:
+    explicit Handle(Value value)
+        : m_value(value)
+    {
+    }
+
+    explicit Handle(Value value, Cell* cell)
+        : m_value(value)
+        , m_handle(Handle<Cell>::create(cell))
+    {
+    }
+
+    Value m_value;
+    Handle<Cell> m_handle;
+};
+
+inline Handle<Value> make_handle(Value value)
+{
+    return Handle<Value>::create(value);
+}
+
 }