settings: Flatten code actions formatters object (#39375)

Closes #ISSUE

Release Notes:

- settings: Changed code action format in `formatter` and
`format_on_save` settings.

**Previous format:**
```
{
  "code_actions": {
    "source.organizeImports": true,
    "source.fixAll": true
  }
}
```

**New format:**
```
[
  {"code_action": "source.organizeImports"},
  {"code_action": "source.fixAll"}
]
```

After #39246, code actions run sequentially in order. The structure now
reflects this and aligns with other formatter options (e.g., language
servers).

Both the `formatter` and `format_on_save` settings will be
auto-migrated.

crates/editor/src/editor_tests.rs

@@ -11874,14 +11874,8 @@ async fn test_multiple_formatters(cx: &mut TestAppContext) {
         settings.defaults.remove_trailing_whitespace_on_save = Some(true);
         settings.defaults.formatter = Some(SelectedFormatter::List(FormatterList::Vec(vec![
             Formatter::LanguageServer { name: None },
-            Formatter::CodeActions(
-                [
-                    ("code-action-1".into(), true),
-                    ("code-action-2".into(), true),
-                ]
-                .into_iter()
-                .collect(),
-            ),
+            Formatter::CodeAction("code-action-1".into()),
+            Formatter::CodeAction("code-action-2".into()),
         ])))
     });