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

docs/src/configuring-zed.md

@@ -634,7 +634,8 @@ List of `string` values
   "close_position": "right",
   "file_icons": false,
   "git_status": false,
-  "activate_on_close": "history"
+  "activate_on_close": "history",
+  "always_show_close_button": false
 },
 ```
 
@@ -698,6 +699,12 @@ List of `string` values
 }
 ```
 
+### Always show the close button
+
+- Description: Whether to always show the close button on tabs.
+- Setting: `always_show_close_button`
+- Default: `false`
+
 ## Editor Toolbar
 
 - Description: Whether or not to show various elements in the editor toolbar.