Use extra small label for tab descriptions (#3797)

This PR adjusts the size of the tab descriptions to use the extra small
label size.

This makes it easier to distinguish between the filename and the
description:

<img width="320" alt="Screenshot 2023-12-22 at 6 49 11 PM"
src="https://github.com/zed-industries/zed/assets/1486634/451d4cd0-666c-43c2-b5df-6d9cf91dacda">

It's also more in line with how things look in Zed1.

Release Notes:

- N/A

crates/ui2/src/components/button/toggle_button.rs

@@ -122,7 +122,7 @@ impl RenderOnce for ToggleButton {
             .child(
                 Label::new(self.label)
                     .color(label_color)
-                    .line_height_style(LineHeightStyle::UILabel),
+                    .line_height_style(LineHeightStyle::UiLabel),
             )
     }
 }