fancy up prints with cli

[mdsteiner]

e.g., add spinners while fitting

[sgrieder]

The following could be interesting:

• rules in print function --> implemented
• headings in print function --> implemented (with rules)
• spinners while fitting --> doesn't work
• stop, warning, and other messages (cli_alert_info) --> implemented

[sgrieder]

https://cli.r-lib.org/articles/semantic-cli.html