fix(shell): Support conditional format strings for $indicator (#2489)

Previously attempting to use conditional format strings with
`$indicator` would never display an indicator, e.g.:

```toml
[shell]
fish_indicator = ""
bash_indicator = "B "
format = "($indicator )"
disabled = false
```

This would always display an empty string.

Fixes #2474.

src/modules/shell.rs

@@ -28,6 +28,16 @@ pub fn module<'a>(context: &'a Context) -> Option<Module<'a>> {
                 },
                 _ => None,
             })
+            .map(|var| match var {
+                "bash_indicator" => Some(Ok(config.bash_indicator)),
+                "fish_indicator" => Some(Ok(config.fish_indicator)),
+                "zsh_indicator" => Some(Ok(config.zsh_indicator)),
+                "powershell_indicator" => Some(Ok(config.powershell_indicator)),
+                "ion_indicator" => Some(Ok(config.ion_indicator)),
+                "elvish_indicator" => Some(Ok(config.elvish_indicator)),
+                "tcsh_indicator" => Some(Ok(config.tcsh_indicator)),
+                _ => None,
+            })
             .parse(None)
     });
 
@@ -237,4 +247,36 @@ mod tests {
 
         assert_eq!(expected, actual);
     }
+
+    #[test]
+    fn test_custom_format_conditional_indicator_match() {
+        let expected = Some(format!("{} ", "B"));
+        let actual = ModuleRenderer::new("shell")
+            .shell(Shell::Bash)
+            .config(toml::toml! {
+                [shell]
+                bash_indicator = "B"
+                format = "($bash_indicator )"
+                disabled = false
+            })
+            .collect();
+
+        assert_eq!(expected, actual);
+    }
+
+    #[test]
+    fn test_custom_format_conditional_indicator_no_match() {
+        let expected = None;
+        let actual = ModuleRenderer::new("shell")
+            .shell(Shell::Fish)
+            .config(toml::toml! {
+                [shell]
+                bash_indicator = "B"
+                format = "($indicator )"
+                disabled = false
+            })
+            .collect();
+
+        assert_eq!(expected, actual);
+    }
 }