Z 1618/extend comments (#2585)

Fixes Z-1618. In the current state, this only works for line comments
such as `//` (and whatever's set in `{language}.toml` as a
line_comment).

Release Notes:

- Comments are now extended on new line.

crates/editor/src/editor_tests.rs

@@ -1719,6 +1719,33 @@ async fn test_newline_below(cx: &mut gpui::TestAppContext) {
     "});
 }
 
+#[gpui::test]
+async fn test_newline_comments(cx: &mut gpui::TestAppContext) {
+    init_test(cx, |settings| {
+        settings.defaults.tab_size = NonZeroU32::new(4)
+    });
+
+    let language = Arc::new(Language::new(
+        LanguageConfig {
+            line_comment: Some("//".into()),
+            ..LanguageConfig::default()
+        },
+        None,
+    ));
+
+    let mut cx = EditorTestContext::new(cx).await;
+    cx.update_buffer(|buffer, cx| buffer.set_language(Some(language), cx));
+    cx.set_state(indoc! {"
+        // Fooˇ
+    "});
+
+    cx.update_editor(|e, cx| e.newline(&Newline, cx));
+    cx.assert_editor_state(indoc! {"
+        // Foo
+        //ˇ
+    "});
+}
+
 #[gpui::test]
 fn test_insert_with_old_selections(cx: &mut TestAppContext) {
     init_test(cx, |_| {});