Add `fromDirectional` factory to `RelativeRect` for when an app needs to consider both TextDirection.ltr and TextDirection.rtl.

```
RelativeRect.fromDirectional(
    textDirection: textDirection,
    start: 10.0,
    top: 20.0,
    end: 30.0,
    bottom: 40.0,
);
```

Addresses https://github.com/flutter/flutter/issues/107058.