LibJS: Generalize PromiseAllResolveElementFunction common functionality

The element-resolving functions on the Promise constructor are all very
similar. To prepare for more of these functions to be implemented, break
out common parts into a base class.

Userland/Libraries/LibJS/Runtime/PromiseResolvingElementFunctions.h

@@ -0,0 +1,77 @@
+/*
+ * Copyright (c) 2021, Tim Flynn <trflynn89@pm.me>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibJS/Runtime/NativeFunction.h>
+#include <LibJS/Runtime/PromiseReaction.h>
+
+namespace JS {
+
+struct RemainingElements final : public Cell {
+    RemainingElements() = default;
+
+    explicit RemainingElements(u64 initial_value)
+        : value(initial_value)
+    {
+    }
+
+    virtual const char* class_name() const override { return "RemainingElements"; }
+
+    u64 value { 0 };
+};
+
+struct PromiseValueList final : public Cell {
+    PromiseValueList()
+        : values(heap())
+    {
+    }
+
+    virtual const char* class_name() const override { return "PromiseValueList"; }
+
+    MarkedValueList values;
+};
+
+class PromiseResolvingElementFunction : public NativeFunction {
+    JS_OBJECT(PromiseResolvingFunction, NativeFunction);
+
+public:
+    virtual void initialize(GlobalObject&) override;
+    virtual ~PromiseResolvingElementFunction() override = default;
+
+    virtual Value call() override;
+
+protected:
+    explicit PromiseResolvingElementFunction(size_t, PromiseValueList&, PromiseCapability, RemainingElements&, Object& prototype);
+
+    virtual Value resolve_element() = 0;
+
+    size_t m_index { 0 };
+    PromiseValueList& m_values;
+    PromiseCapability m_capability;
+    RemainingElements& m_remaining_elements;
+
+private:
+    virtual void visit_edges(Visitor&) override;
+
+    bool m_already_called { false };
+};
+
+// 27.2.4.1.3 Promise.all Resolve Element Functions, https://tc39.es/ecma262/#sec-promise.all-resolve-element-functions
+class PromiseAllResolveElementFunction final : public PromiseResolvingElementFunction {
+    JS_OBJECT(PromiseResolvingFunction, NativeFunction);
+
+public:
+    static PromiseAllResolveElementFunction* create(GlobalObject&, size_t, PromiseValueList&, PromiseCapability, RemainingElements&);
+
+    explicit PromiseAllResolveElementFunction(size_t, PromiseValueList&, PromiseCapability, RemainingElements&, Object& prototype);
+    virtual ~PromiseAllResolveElementFunction() override = default;
+
+private:
+    virtual Value resolve_element() override;
+};
+
+}