Remove prism as a runtime dependency

[marcandre]

Fixes #282

[koic]

It is mentioned below, it seems better to hold off on making a decision to remove it for now: https://github.com/rubocop/rubocop-ast/issues/282#issuecomment-1972341899

If the decision is made to remove it, it would likely require adjustments not only to the removing dependency, also to issues surrounding Prism's loading and documentation. It might be better if I handle it. Thank you.

[koic]

I've opened #285. It addresses users who don't have Prism installed, indicating through warning and documentation that Prism needs to be installed separately.

[bbatsov]

Addressed by #285.