LibJS: Add initial implementation for SharedArrayBuffer

None of the actual sharing is implemented yet, but this is enough for
most basic functionality.

Diff Tests:
    +260 ✅    -262 ❌   +2 💀

Userland/Libraries/LibJS/CMakeLists.txt

@@ -196,6 +196,8 @@ set(SOURCES
     Runtime/ShadowRealmConstructor.cpp
     Runtime/ShadowRealmPrototype.cpp
     Runtime/Shape.cpp
+    Runtime/SharedArrayBufferConstructor.cpp
+    Runtime/SharedArrayBufferPrototype.cpp
     Runtime/StringConstructor.cpp
     Runtime/StringIterator.cpp
     Runtime/StringIteratorPrototype.cpp