Reconsider requiring textStyle property for AnimatedText subclasses

[awhitford]

The textStyle property is required for AnimatedText subclasses. It can lead to a bunch of repetition.

Does it make sense to leverage the DefaultTextStyle widget? Could that simplify the code?

Note that being able to provide a unique textStyle is still valuable. However, when you are specifying the same value for each AnimatedText instance, it becomes verbose and repetitive. The expectation is that when no explicit textStyle is specified, it would fallback to the default text style, and the DefaultTextStyle widget could be used to wrap the whole AnimatedTextKit if you want to apply a consistent style across animations.

Success would mean:

• we could remove the required modifier for textStyle across animations
• usage and sample code could be leaner (less lines of code)

[awhitford]

Note that I had this idea from PR #216, resolving #211.