Enable clippy::single_char_pattern (#8727)

This PR enables the
[`clippy::single_char_pattern`](https://rust-lang.github.io/rust-clippy/master/index.html#/single_char_pattern)
rule and fixes the outstanding violations.

Release Notes:

- N/A

crates/workspace/src/pane.rs

@@ -2586,8 +2586,8 @@ mod tests {
 
             let mut index = 0;
             let items = labels.map(|mut label| {
-                if label.ends_with("*") {
-                    label = label.trim_end_matches("*");
+                if label.ends_with('*') {
+                    label = label.trim_end_matches('*');
                     active_item_index = index;
                 }