Selectable text

Can you guys add a selectable text feature?

Look at the existing text animation widgets in the package to understand how text is rendered. Investigate Flutter's SelectableText widget and how to integrate selection capabilities into animated text components. Determine which animations support selection and what 'done' looks like: a user can select the animated text with the system text selection interface.

索引モデルが issue の本文から書いたものです。