Pane: Add tab pinning (#17426)

Closes #5046

Release Notes:

- Added "Pin/Unpin Tab" action to the workspace, assistant and terminal
tabs.

---------

Co-authored-by: Danilo <danilo@zed.dev>

crates/ui/src/components/stories/tab.rs

@@ -28,6 +28,7 @@ impl Render for TabStory {
                     Tab::new("tab_1")
                         .end_slot(
                             IconButton::new("close_button", IconName::Close)
+                                .visible_on_hover("")
                                 .shape(IconButtonShape::Square)
                                 .icon_color(Color::Muted)
                                 .size(ButtonSize::None)

crates/ui/src/components/tab.rs

@@ -120,11 +120,7 @@ impl RenderOnce for Tab {
         let (start_slot, end_slot) = {
             let start_slot = h_flex().size_3().justify_center().children(self.start_slot);
 
-            let end_slot = h_flex()
-                .size_3()
-                .justify_center()
-                .visible_on_hover("")
-                .children(self.end_slot);
+            let end_slot = h_flex().size_3().justify_center().children(self.end_slot);
 
             match self.close_side {
                 TabCloseSide::End => (start_slot, end_slot),