LibJS: Add Proxy objects

Includes all traps except the following: [[Call]], [[Construct]],
[[OwnPropertyKeys]].

An important implication of this commit is that any call to any virtual
Object method has the potential to throw an exception. These methods
were not checked in this commit -- a future commit will have to protect
these various method calls throughout the codebase.

Libraries/LibJS/CMakeLists.txt

@@ -41,6 +41,10 @@ set(SOURCES
     Runtime/Object.cpp
     Runtime/ObjectPrototype.cpp
     Runtime/PrimitiveString.cpp
+    Runtime/PropertyAttributes.cpp
+    Runtime/ProxyConstructor.cpp
+    Runtime/ProxyObject.cpp
+    Runtime/ProxyPrototype.cpp
     Runtime/Reference.cpp
     Runtime/ReflectObject.cpp
     Runtime/ScriptFunction.cpp