feat: starship prompt

Kind of touches on #356 by integrating the Starship prompt directly into the shell.

Not finished yet and has surfaced a potential bug in rustyline anyway. It depends on https://github.com/starship/starship/pull/509 being merged so the Starship prompt can be used as a library.

I could have tackled #356 completely and implemented a full custom prompt feature but I felt this was a simpler approach given that Starship is both written in Rust so shelling out isn't necessary and it already has a bunch of useful features built in.

However, I would understand if it would be preferable to just scrap integrating Starship directly and instead implement a custom prompt system which would facilitate simply shelling out to Starship.

src/cli.rs

@@ -365,14 +365,26 @@ pub async fn cli() -> Result<(), Box<dyn Error>> {
         // Redefine Ctrl-D to same command as Ctrl-C
         rl.bind_sequence(rustyline::KeyPress::Ctrl('D'), rustyline::Cmd::Interrupt);
 
-        let prompt = &format!(
-            "{}{}> ",
-            cwd,
-            match current_branch() {
-                Some(s) => format!("({})", s),
-                None => "".to_string(),
+        let prompt = {
+            #[cfg(feature = "starship-prompt")]
+            {
+                &starship::print::get_prompt(starship::context::Context::new_with_dir(
+                    clap::ArgMatches::default(),
+                    cwd,
+                ))
             }
-        );
+            #[cfg(not(feature = "starship-prompt"))]
+            {
+                &format!(
+                    "{}{}> ",
+                    cwd,
+                    match current_branch() {
+                        Some(s) => format!("({})", s),
+                        None => "".to_string(),
+                    }
+                )
+            }
+        };
         let mut initial_command = Some(String::new());
         let mut readline = Err(ReadlineError::Eof);
         while let Some(ref cmd) = initial_command {