Add toggle case command (#28415)

A small addition for those coming from JetBrain's IDEs. A behavioral
detail: when any upper case character is detected, the command defaults
to toggling to lower case.

> Note that when you apply the toggle case action to the CamelCase name
format, IntelliJ IDEA converts the name to the lower case.


https://www.jetbrains.com/help/idea/working-with-source-code.html#edit_code_fragments

Release Notes:

- Added an `editor: toggle case` command. Use `cmd-shift-u` for macOS
and `ctrl-shift-u` for Linux, when using the `JetBrains` keymap.

crates/editor/src/editor_tests.rs

@@ -3875,6 +3875,41 @@ async fn test_manipulate_lines_with_multi_selection(cx: &mut TestAppContext) {
     "});
 }
 
+#[gpui::test]
+async fn test_toggle_case(cx: &mut TestAppContext) {
+    init_test(cx, |_| {});
+
+    let mut cx = EditorTestContext::new(cx).await;
+
+    // If all lower case -> upper case
+    cx.set_state(indoc! {"
+        «hello worldˇ»
+    "});
+    cx.update_editor(|e, window, cx| e.toggle_case(&ToggleCase, window, cx));
+    cx.assert_editor_state(indoc! {"
+        «HELLO WORLDˇ»
+    "});
+
+    // If all upper case -> lower case
+    cx.set_state(indoc! {"
+        «HELLO WORLDˇ»
+    "});
+    cx.update_editor(|e, window, cx| e.toggle_case(&ToggleCase, window, cx));
+    cx.assert_editor_state(indoc! {"
+        «hello worldˇ»
+    "});
+
+    // If any upper case characters are identified -> lower case
+    // This matches JetBrains IDEs
+    cx.set_state(indoc! {"
+        «hEllo worldˇ»
+    "});
+    cx.update_editor(|e, window, cx| e.toggle_case(&ToggleCase, window, cx));
+    cx.assert_editor_state(indoc! {"
+        «hello worldˇ»
+    "});
+}
+
 #[gpui::test]
 async fn test_manipulate_text(cx: &mut TestAppContext) {
     init_test(cx, |_| {});