Add padding extensions

Create easy extensions for padding in a flutter project.

Example:

```dart
Text('apple').paddingSmall()
```

Provide helpers to customise small, medium large as per the design guidelines for the app.

```dart
/// tiny: 2.0, small: 4.0, regular 8.0, medium: 16.0, large: 32.0
WidgetXHelper.setPaddingSpec(grid_spacer: 4.0);
```