LibJS: Implement constructor/non-constructor function calls

This adds Function::construct() for constructor function calls via `new`
keyword. NativeFunction doesn't have constructor behaviour by default,
ScriptFunction simply calls call() in construct()

Libraries/LibJS/Runtime/DateConstructor.cpp

@@ -46,6 +46,14 @@ DateConstructor::~DateConstructor()
 }
 
 Value DateConstructor::call(Interpreter& interpreter)
+{
+    auto* date = static_cast<Date*>(construct(interpreter).as_object());
+    if (!date)
+        return {};
+    return js_string(interpreter.heap(), date->string());
+}
+
+Value DateConstructor::construct(Interpreter& interpreter)
 {
     // TODO: Support args
     struct timeval tv;