Add `slim` gem

## Reasoning

Haml is similar and has more downloads, partially because it's older.  I've used it before, but it's been a while.

I've used Slim more recently and I'd like to try some of the plugins like for logicless templates:
https://github.com/slim-template/slim/blob/main/doc/logic_less.md

Slim also doesn't use `%` for bare HTML tags