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/Makefile

@@ -8,6 +8,7 @@ OBJS = \
     Interpreter.o \
     Lexer.o \
     NativeFunction.o \
+    NativeProperty.o \
     Object.o \
     ObjectPrototype.o \
     Parser.o \