LibJS: Add a mechanism for callback-based object properties

This patch adds NativeProperty, which can be used to implement object
properties that have C++ getters and/or setters.

Use this to move String.prototype.length to its correct place. :^)

Libraries/LibJS/StringObject.cpp

@@ -37,7 +37,6 @@ StringObject::StringObject(PrimitiveString* string)
     : m_string(string)
 {
     set_prototype(interpreter().string_prototype());
-    put("length", Value(static_cast<i32>(m_string->string().length())));
 }
 
 StringObject::~StringObject()