LibJS: Add a simple reference-counted UTF-16 string

To help alleviate memory usage when creating and copying large strings,
create a simple wrapper around a Vector<u16> to reference count UTF-16
strings.

Userland/Libraries/LibJS/CMakeLists.txt

@@ -157,6 +157,7 @@ set(SOURCES
     Runtime/TypedArray.cpp
     Runtime/TypedArrayConstructor.cpp
     Runtime/TypedArrayPrototype.cpp
+    Runtime/Utf16String.cpp
     Runtime/Value.cpp
     Runtime/VM.cpp
     Runtime/WeakContainer.cpp