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/AgentObject.cpp

@@ -0,0 +1,46 @@
+/*
+ * Copyright (c) 2021-2022, Linus Groh <linusg@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <AK/Time.h>
+#include <LibJS/Contrib/Test262/AgentObject.h>
+#include <LibJS/Runtime/GlobalObject.h>
+#include <LibJS/Runtime/Object.h>
+#include <unistd.h>
+
+namespace JS::Test262 {
+
+AgentObject::AgentObject(JS::GlobalObject& global_object)
+    : Object(Object::ConstructWithoutPrototypeTag::Tag, global_object)
+{
+}
+
+void AgentObject::initialize(JS::GlobalObject& global_object)
+{
+    Base::initialize(global_object);
+
+    u8 attr = Attribute::Writable | Attribute::Configurable;
+    define_native_function("monotonicNow", monotonic_now, 0, attr);
+    define_native_function("sleep", sleep, 1, attr);
+    // TODO: broadcast
+    // TODO: getReport
+    // TODO: start
+}
+
+JS_DEFINE_NATIVE_FUNCTION(AgentObject::monotonic_now)
+{
+    auto time = Time::now_monotonic();
+    auto milliseconds = time.to_milliseconds();
+    return Value(static_cast<double>(milliseconds));
+}
+
+JS_DEFINE_NATIVE_FUNCTION(AgentObject::sleep)
+{
+    auto milliseconds = TRY(vm.argument(0).to_i32(global_object));
+    ::usleep(milliseconds * 1000);
+    return js_undefined();
+}
+
+}