LibJS: Import C++ sources from libjs-test262 :^)

This commit upstreams most of the C++ bits of the LibJS test262 runner
at https://github.com/linusg/libjs-test262/, specifically everything but
the main.cpp file serving as the actual executable.
Since all of these are just regular JS objects, I opted to put them in
LibJS itself, in a new Contrib/ directory like many other projects have
one. Other code that can end up there in the future is the runtime for
esvu, which might even share some functionality with test262's $262
object.

The code has been copied verbatim, and only a small number of changes
have been made:

- Putting everything into the JS::Test262 namespace
- Removing now redundant JS namespace prefixes
- Updating includes to use absolute <LibJS/...> paths
- Updating the SPDX-License-Identifier comments from MIT to BSD-2-Clause

I gained permission to change the license and upstream these changes
from all the major contributors to this code: Ali, Andrew, David, Idan.

The removal of the code from the source repository is here:
https://github.com/linusg/libjs-test262/pull/54

This is only the first step, the goal is to eventually upstream the
actual libjs-test262-runner executable and supporting Python scripts
into SerenityOS as well.

Userland/Libraries/LibJS/Contrib/Test262/$262Object.cpp

@@ -0,0 +1,88 @@
+/*
+ * Copyright (c) 2021, Linus Groh <linusg@serenityos.org>
+ * Copyright (c) 2021, Idan Horowitz <idan.horowitz@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibJS/Contrib/Test262/$262Object.h>
+#include <LibJS/Contrib/Test262/AgentObject.h>
+#include <LibJS/Contrib/Test262/GlobalObject.h>
+#include <LibJS/Contrib/Test262/IsHTMLDDA.h>
+#include <LibJS/Heap/Cell.h>
+#include <LibJS/Interpreter.h>
+#include <LibJS/Runtime/ArrayBuffer.h>
+#include <LibJS/Runtime/GlobalObject.h>
+#include <LibJS/Runtime/Object.h>
+#include <LibJS/Script.h>
+
+namespace JS::Test262 {
+
+$262Object::$262Object(JS::GlobalObject& global_object)
+    : Object(Object::ConstructWithoutPrototypeTag::Tag, global_object)
+{
+}
+
+void $262Object::initialize(JS::GlobalObject& global_object)
+{
+    Base::initialize(global_object);
+
+    m_agent = vm().heap().allocate<AgentObject>(global_object, global_object);
+    m_is_htmldda = vm().heap().allocate<IsHTMLDDA>(global_object, global_object);
+
+    u8 attr = Attribute::Writable | Attribute::Configurable;
+    define_native_function("clearKeptObjects", clear_kept_objects, 0, attr);
+    define_native_function("createRealm", create_realm, 0, attr);
+    define_native_function("detachArrayBuffer", detach_array_buffer, 1, attr);
+    define_native_function("evalScript", eval_script, 1, attr);
+
+    define_direct_property("agent", m_agent, attr);
+    define_direct_property("gc", global_object.get_without_side_effects("gc"), attr);
+    define_direct_property("global", &global_object, attr);
+    define_direct_property("IsHTMLDDA", m_is_htmldda, attr);
+}
+
+void $262Object::visit_edges(Cell::Visitor& visitor)
+{
+    Base::visit_edges(visitor);
+    visitor.visit(m_agent);
+    visitor.visit(m_is_htmldda);
+}
+
+JS_DEFINE_NATIVE_FUNCTION($262Object::clear_kept_objects)
+{
+    vm.finish_execution_generation();
+    return js_undefined();
+}
+
+JS_DEFINE_NATIVE_FUNCTION($262Object::create_realm)
+{
+    auto realm = vm.heap().allocate_without_global_object<GlobalObject>();
+    realm->initialize_global_object();
+    return Value(realm->$262());
+}
+
+// 25.1.2.3 DetachArrayBuffer, https://tc39.es/ecma262/#sec-detacharraybuffer
+JS_DEFINE_NATIVE_FUNCTION($262Object::detach_array_buffer)
+{
+    auto array_buffer = vm.argument(0);
+    if (!array_buffer.is_object() || !is<ArrayBuffer>(array_buffer.as_object()))
+        return vm.throw_completion<TypeError>(global_object);
+    auto& array_buffer_object = static_cast<ArrayBuffer&>(array_buffer.as_object());
+    if (!same_value(array_buffer_object.detach_key(), vm.argument(1)))
+        return vm.throw_completion<TypeError>(global_object);
+    array_buffer_object.detach_buffer();
+    return js_null();
+}
+
+JS_DEFINE_NATIVE_FUNCTION($262Object::eval_script)
+{
+    auto source = TRY(vm.argument(0).to_string(global_object));
+    auto script_or_error = Script::parse(source, *vm.current_realm());
+    if (script_or_error.is_error())
+        return vm.throw_completion<SyntaxError>(global_object, script_or_error.error()[0].to_string());
+    TRY(vm.interpreter().run(script_or_error.value()));
+    return js_undefined();
+}
+
+}