Initial wait

**Is your feature request related to a problem? Please describe.**
Typing animations are often used to mimic human behaviour(as in chat bots). Ability to add an initial wait before the animation start would be great. Is there a built-in way to do this?

**Describe the solution you'd like**
Ability to add initial wait to text animation.
```dart
...
pause: const Duration(milliseconds: 1000),
initialPause: const Duration(milliseconds: 1000),
...
```

Look at the existing animation classes in the package, such as TypewriterAnimatedTextKit, to understand how the 'pause' parameter works. The goal is to add an 'initialPause' parameter to the animation configuration. Check the base classes for where the animation lifecycle starts and where the initial delay could be inserted. Run the example app to see current behavior and test the new feature.

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