Remove auto-width editor type (#34438)

Closes #34044

`EditorMode::SingleLine { auto_width: true }` was only used for the
title editor in the rules library, and following
https://github.com/zed-industries/zed/pull/31994 we can replace that
with a normal single-line editor without problems. The auto-width editor
was interacting badly with the recently-added newline visualization
code, causing a panic during layout---by switching it to
`Editor::single_line` the newline visualization works there too.

Release Notes:

- Fixed a panic that could occur when opening the rules library.

---------

Co-authored-by: Finn <finn@zed.dev>

crates/rules_library/src/rules_library.rs

@@ -611,7 +611,7 @@ impl RulesLibrary {
                 this.update_in(cx, |this, window, cx| match rule {
                     Ok(rule) => {
                         let title_editor = cx.new(|cx| {
-                            let mut editor = Editor::auto_width(window, cx);
+                            let mut editor = Editor::single_line(window, cx);
                             editor.set_placeholder_text("Untitled", cx);
                             editor.set_text(rule_metadata.title.unwrap_or_default(), window, cx);
                             if prompt_id.is_built_in() {