refactor: create library with simple entrypoint in main

Create a library entrypoint 'src/lib.rs' and refactor binary entrypoint
'src/main.rs' accordingly.

src/main.rs

@@ -1,11 +1,7 @@
-#![allow(unused)]
-
-mod cli;
-mod scripts;
+use ahab::{cli, scripts};
 
 use anyhow::Result;
 use clap::Parser;
-use std::env;
 
 fn main() -> Result<()> {
     // always load dotenv on start