LibJS: Introduce NativeJavaScriptBackedFunction

This hosts the ability to compile and run JavaScript to implement
native functions. This is particularly useful for any native function
that is not a normal function, for example async functions such as
Array.fromAsync, which require yielding.

These functions are not allowed to observe anything from outside their
environment. Any global identifiers will instead be assumed to be a
reference to an abstract operation or a constant. The generator will
inject the appropriate bytecode if the name of the global identifier
matches a known name. Anything else will cause a code generation error.

Libraries/LibJS/Runtime/NativeJavaScriptBackedFunction.h

@@ -0,0 +1,43 @@
+/*
+ * Copyright (c) 2025, Luke Wilde <luke@ladybird.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibJS/Runtime/NativeFunction.h>
+#include <LibJS/Runtime/SharedFunctionInstanceData.h>
+
+namespace JS {
+
+class NativeJavaScriptBackedFunction final : public NativeFunction {
+    JS_OBJECT(NativeJavaScriptBackedFunction, NativeFunction);
+    GC_DECLARE_ALLOCATOR(NativeJavaScriptBackedFunction);
+
+public:
+    static GC::Ref<NativeJavaScriptBackedFunction> create(Realm&, FunctionNode const& function_node, PropertyKey const& name, i32 length);
+
+    virtual ~NativeJavaScriptBackedFunction() override = default;
+
+    virtual void visit_edges(Visitor&) override;
+
+    virtual ThrowCompletionOr<void> get_stack_frame_size(size_t& registers_and_constants_and_locals_count, size_t& argument_count) override;
+
+    virtual ThrowCompletionOr<Value> call() override;
+
+    Bytecode::Executable& bytecode_executable();
+    FunctionKind kind() const;
+    ThisMode this_mode() const;
+
+    virtual bool function_environment_needed() const override;
+    virtual size_t function_environment_bindings_count() const override;
+    virtual bool is_strict_mode() const override;
+
+private:
+    explicit NativeJavaScriptBackedFunction(GC::Ref<SharedFunctionInstanceData const> shared_function_instance_data, Object& prototype);
+
+    GC::Ref<SharedFunctionInstanceData const> m_shared_function_instance_data;
+};
+
+}