Add option to activate left neighbour tab on tab close (#21800)

Closes #21738

Release Notes:

- Added `left_neighbour` option to the `tabs.activate_on_close` setting
to activate the left adjacent tab on tab close.

docs/src/configuring-zed.md

@@ -691,7 +691,7 @@ List of `string` values
 }
 ```
 
-2. Activate the neighbour tab (prefers the right one, if present):
+2. Activate the right neighbour tab if present:
 
 ```json
 {
@@ -699,6 +699,14 @@ List of `string` values
 }
 ```
 
+3. Activate the left neighbour tab if present:
+
+```json
+{
+  "activate_on_close": "left_neighbour"
+}
+```
+
 ### Always show the close button
 
 - Description: Whether to always show the close button on tabs.