Add text_color helper for tab contents (#14737)

This PR adds a `text_color` method to `TabContentParams` to more easily
compute the text color to be used for tab contents.

This consolidates a number of conditionals that were scattered all over
the place to give us a singular source of truth for these colors.

Release Notes:

- N/A

crates/terminal_view/src/terminal_view.rs

@@ -994,11 +994,7 @@ impl Item for TerminalView {
                         )
                     }),
             )
-            .child(Label::new(title).color(if params.selected {
-                Color::Default
-            } else {
-                Color::Muted
-            }))
+            .child(Label::new(title).color(params.text_color()))
             .into_any()
     }