Prompt: Color of the git branch

[certik]

I used to use red color for years. But the default on macOS seems to be green:

The first is shell, the second is zsh. I actually kind of like the green color. So I wonder if we should use it by default.

We need to enable configuring the prompt, so that users can set it up any way they like.

[certik]

Here is how to achieve the green color:

--- a/crates/shell/src/main.rs
+++ b/crates/shell/src/main.rs
@@ -115,7 +115,7 @@ async fn interactive(state: Option<ShellState>, norc: bool) -> miette::Result<()
             };

             let prompt = format!("{}{git_branch}$ ", display_cwd);
-            let color_prompt = format!("\x1b[34m{}\x1b[31m{git_branch}\x1b[0m$ ", display_cwd);
+            let color_prompt = format!("\x1b[34m{}\x1b[32m{git_branch}\x1b[0m$ ", display_cwd);
             rl.helper_mut().unwrap().colored_prompt = color_prompt;
             rl.readline(&prompt)
         };

[certik]

I opened up #200 for the general solution to this.