[ Widget Preview ] Remove `WidgetPreview` in favor of using annotation properties (#165500)

This change reworks how users define previews in their code, expands the
number of valid 'functions' that can be used to create previews, and
allows for specifying a 'wrapper' function to wrap the previewed widget
with

The `WidgetPreview` class has been removed from the framework, with its
properties being added to the `Preview` annotation class instead to
remove some boilerplate from the preview declaration workflow.

Before:

```dart
@Preview()
List<WidgetPreview> previews() => <WidgetPreview>[
      WidgetPreview(
        name: 'Top-level preview',
        child: Text('Foo'),
      ),
  ];
```

After:

```dart
@Preview(name: 'Top-level preview')
Widget previews() => Text('Foo');
```

Previews can now be defined using top-level functions, constructors and
factories which take no arguments, and static methods within classes:

Examples:

```dart
@Preview(name: 'Top-level preview')
Widget previews() => Text('Foo');

class MyWidget extends StatelessWidget {
  @Preview(name: 'Constructor preview')
  MyWidget.preview();

  @Preview(name: 'Factory preview')
  factory MyWidget.factoryPreview() => MyWidget.preview();

  @Preview(name: 'Static preview')
  static Widget previewStatic() => Text('Static');

  @override
  Widget build(BuildContext context) {
    return Text('MyWidget');
  }
}
```

Users can also provide a `wrapper` function with the signature `Widget
Function(Widget)` to easily wrap previewed widget with shared
bootstrapping logic.

Example:

```dart
Widget testWrapper(Widget child) {
  return Provider<int>.value(
    value: 42,
    child: child,
  );
}

@Preview(name: 'Preview with wrapper', wrapper: testWrapper)
Widget preview() {
  return Text('Attributes');
}
```

Which is effectively the same as:

```dart
@Preview(name: 'Preview with wrapper')
Widget preview() {
  return Provider<int>.value(
    value: 42,
    child: Text('Attributes'),
  );
}
```

Finally, for situations where a `BuildContext` is needed, users can
return a `WidgetBuilder` from their preview function:

```dart
@Preview('Builder preview')
WidgetBuilder builderPreview() {
  return (BuildContext context) {
    // TODO: retrieve state from context.
    return Text('Foo');
  };
}
```
https://dart.googlesource.com/external/github.com/flutter/flutter/+/41c427c6de863e3cc3cfb71649b868940a97d8b7