Some suggested improvements

[jor-alv]

Hi there @sogoagain!

I have been playing around with scrambling-text-js (it looks great) and would love to help you make some improvements to it, namely:

• Add some additional built-in character choices,
• Add some additional options for users to more easily customize and control the animation,
• Remove duplicate variable initializations and make other minor clean-ups.

I have created a new branch to keep changes separated from the main branch and have split the proposed changes into five different commits to make them easier for you to review. None of them are breaking changes and I have tried to make commit messages as self-explanatory as possible.

Also, I have run both the original tests and some additional tests I have added, and run the animation locally, and everything works OK.

Cheers!