Add primitives

[esDotDev]

Added some value types for easily rebuilding the view when data changes.

• ReactiveInt
• ReactiveBool
• ReactiveString
• ReactiveDouble
• ReactiveValue<T>

Address this issue: #2

Tweaked the example to make use of the bool reactive for showPassword:

class ReactiveLoginLogic extends Reactive {
  ReactiveLoginLogic(ReactiveHost host) : super(host);
  late final ReactiveTextEditingController _email = ReactiveTextEditingController(host);
  late final ReactiveTextEditingController _password = ReactiveTextEditingController(host);
  late final ReactiveBool _showPasswordReactive = ReactiveBool(host); // rebuilds when changed

  TextEditingController get emailCtrl => _email.ctrl;
  TextEditingController get passwordCtrl => _password.ctrl;
  bool get showPassword => _showPasswordReactive.value;

  // Login logic
  void submit() {}

  void toggleShowPassword() => _showPasswordReactive.value = !showPassword;
}