refactor!: separate lib and cli

[comfysage]

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[uncenter]

Thanks!

[comfysage]

kitty can currently be called like this:

use kittysay::{KITTY, generate};

pub fn main() {
  let format_opts = FormatOpts {
    think: false,
    width: 80,
  };
  println!(format!("{}{}", generate("<3", format_opts), KITTY));
}

or we could add a kittysay::print fn:

pub fn print(message: &str, format_opts: FormatOptions) -> String {
  format!("{}{}", generate(message, format_opts), KITTY)
}

[comfysage]

print fn pr in #22