LibJS: Split Function into subclasses NativeFunction and ScriptFunction

Both types of functions are now Function and implement calling via:

    virtual Value call(Interpreter&, Vector<Value> arguments);

This removes the need for CallExpression::execute() to care about which
kind of function it's calling. :^)

Libraries/LibJS/Makefile

@@ -11,6 +11,7 @@ OBJS = \
     Object.o \
     Parser.o \
     PrimitiveString.o \
+    ScriptFunction.o \
     StringObject.o \
     Token.o \
     Value.o