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/Function.cpp

@@ -29,10 +29,7 @@
 
 namespace JS {
 
-Function::Function(String name, const ScopeNode& body, Vector<String> parameters)
-    : m_name(move(name))
-    , m_body(body)
-    , m_parameters(move(parameters))
+Function::Function()
 {
 }