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/CMakeLists.txt

@@ -15,6 +15,10 @@ set(SOURCES
     Bytecode/Pass/UnifySameBlocks.cpp
     Bytecode/StringTable.cpp
     Console.cpp
+    Contrib/Test262/$262Object.cpp
+    Contrib/Test262/AgentObject.cpp
+    Contrib/Test262/GlobalObject.cpp
+    Contrib/Test262/IsHTMLDDA.cpp
     CyclicModule.cpp
     Heap/BlockAllocator.cpp
     Heap/CellAllocator.cpp