Add animation RemoveTextLetterByLetter

[sparshg]

Overview: What does this pull request change?

Added animation RemoveTextLetterByLetter

Motivation and Explanation: Why and how do your changes improve the library?

Before you could not remove the text letter by letter even using this: AddTextLetterByLetter(t8, rate_func=there_and_back)

Now the above works, or something like following will also work RemoveTextLetterByLetter(t8, rate_func=rush_from)

https://user-images.githubusercontent.com/43041139/179393852-350b255e-485e-430c-8b51-8160d6ae0e47.mp4

Links to added or changed documentation pages

Further Information and Comments

Reviewer Checklist

• [ ] The PR title is descriptive enough for the changelog, and the PR is labeled correctly
• [ ] If applicable: newly added non-private functions and classes have a docstring including a short summary and a PARAMETERS section
• [ ] If applicable: newly added functions and classes are tested