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.

crates/workspace/src/item.rs

@@ -71,11 +71,12 @@ pub enum ShowDiagnostics {
 }
 
 #[derive(Clone, Default, Serialize, Deserialize, JsonSchema)]
-#[serde(rename_all = "lowercase")]
+#[serde(rename_all = "snake_case")]
 pub enum ActivateOnClose {
     #[default]
     History,
     Neighbour,
+    LeftNeighbour,
 }
 
 #[derive(Clone, Default, Serialize, Deserialize, JsonSchema)]