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/editor2/src/items.rs

@@ -603,7 +603,11 @@ impl Item for Editor {
                 Color::Muted
             }))
             .when_some(description, |this, description| {
-                this.child(Label::new(description).color(Color::Muted))
+                this.child(
+                    Label::new(description)
+                        .size(LabelSize::XSmall)
+                        .color(Color::Muted),
+                )
             })
             .into_any_element()
     }