ui: Add margin style methods to Label and LabelLike (#14032)

This PR adds margin style methods to the `Label` and `LabelLike`
components.

This allows for callers to provide a margin to these components without
needing to introduce a wrapping `div` to do so.

Release Notes:

- N/A

crates/collab_ui/src/collab_panel.rs

@@ -2547,9 +2547,8 @@ impl CollabPanel {
                     .take(FACEPILE_LIMIT)
                     .chain(if extra_count > 0 {
                         Some(
-                            div()
+                            Label::new(format!("+{extra_count}"))
                                 .ml_2()
-                                .child(Label::new(format!("+{extra_count}")))
                                 .into_any_element(),
                         )
                     } else {