Add setting for making the tab's close button always visible (#21352)

Closes https://github.com/zed-industries/zed/issues/20422

<img width="700" alt="Screenshot 2024-11-29 at 22 00 20"
src="https://github.com/user-attachments/assets/4a17d00c-d64f-4b33-97a7-a57766ce6d17">

Release Notes:

- N/A

crates/workspace/src/item.rs

@@ -42,6 +42,7 @@ pub struct ItemSettings {
     pub close_position: ClosePosition,
     pub activate_on_close: ActivateOnClose,
     pub file_icons: bool,
+    pub always_show_close_button: bool,
 }
 
 #[derive(Deserialize)]
@@ -85,6 +86,10 @@ pub struct ItemSettingsContent {
     ///
     /// Default: history
     pub activate_on_close: Option<ActivateOnClose>,
+    /// Whether to always show the close button on tabs.
+    ///
+    /// Default: false
+    always_show_close_button: Option<bool>,
 }
 
 #[derive(Clone, Default, Serialize, Deserialize, JsonSchema)]