vim: Fix comma insertion for newline in JSON

[kurtbuilds]

Closes #18509

I tried to create a test case for this but ran into blocks. Neovim doesn't appear to do this (IntelliJ does), so I couldn't use the NeovimTestCase. I tried using VimTestCase:

#[gpui::test]
    async fn test_comma_insertion(cx: &mut gpui::TestAppContext) {
        // Neovim doesn't do this, so use a Zed-only test
        let mut cx = VimTestContext::new(cx, true).await;
        let doc = r#"{
  "foo": "bar",
  "baz": "qux"ˇ
}"#;
        // cx.set_state(doc, Mode::Normal);
        cx.update_buffer(|b, cx| {
            b.set_text(doc, cx);
            let mut config = LanguageConfig::default();
            config.insert_trailing_commas = true;
            config.tab_size = Some(2.try_into().unwrap());
            b.set_language(Some(std::sync::Arc::new(Language::new(config, None))), cx);
        });
        // cx.set_neovim_option("filetype=json").await;
        let expected = r#"{
  "foo": "bar",
  "baz": "qux",
  ˇ
}"#;
        // cx.set_state(doc, Mode::Normal);
        cx.simulate_keystroke("o");
        assert_eq!(cx.display_text(), expected);

but it doesn't seem to execute the newline action with the cx.simulate_keystroke("o"); line.

Release Notes:

• Zed will insert commas when creating newlines in Vim for JSON/JSONC documents

[ConradIrwin]

@kurtbuilds you might need to make sure you're in normal mode.

That said, I think we should build this primarily in the editor::Newline handler rather than just for o in normal mode

[kurtbuilds]

That makes sense. I'll take another stab at it this weekend :)