LibJS: Add basic support for "with" statements

with statements evaluate an expression and put the result of it at the
"front" of the scope chain. This is implemented by creating a WithScope
object and placing it in front of the VM's current call frame's scope.

Libraries/LibJS/CMakeLists.txt

@@ -76,6 +76,7 @@ set(SOURCES
     Runtime/Uint8ClampedArray.cpp
     Runtime/VM.cpp
     Runtime/Value.cpp
+    Runtime/WithScope.cpp
     Token.cpp
 )