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/IsHTMLDDA.cpp

@@ -0,0 +1,40 @@
+/*
+ * Copyright (c) 2021, Linus Groh <linusg@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibJS/Contrib/Test262/IsHTMLDDA.h>
+#include <LibJS/Runtime/GlobalObject.h>
+
+namespace JS::Test262 {
+
+IsHTMLDDA::IsHTMLDDA(JS::GlobalObject& global_object)
+    // NativeFunction without prototype is currently not possible (only due to the lack of a ctor that supports it)
+    : NativeFunction("IsHTMLDDA", *global_object.function_prototype())
+{
+}
+
+ThrowCompletionOr<Value> IsHTMLDDA::call()
+{
+    auto& vm = this->vm();
+    if (vm.argument_count() == 0)
+        return js_null();
+    if (vm.argument(0).is_string() && vm.argument(0).as_string().string().is_empty())
+        return js_null();
+    // Not sure if this really matters, INTERPRETING.md simply says:
+    // * IsHTMLDDA - (present only in implementations that can provide it) an object that:
+    //   a. has an [[IsHTMLDDA]] internal slot, and
+    //   b. when called with no arguments or with the first argument "" (an empty string) returns null.
+    return js_undefined();
+}
+
+ThrowCompletionOr<Object*> IsHTMLDDA::construct(FunctionObject&)
+{
+    // Not sure if we need to support construction, but ¯\_(ツ)_/¯
+    auto& vm = this->vm();
+    auto& global_object = this->global_object();
+    return vm.throw_completion<TypeError>(global_object, ErrorType::NotAConstructor, "IsHTMLDDA");
+}
+
+}