Strip carriage returns from all text in text::Buffer

* Moving the logic from Rope to text::Buffer makes it easier
  to keep the Rope in sync with the fragment tree.
* Removing carriage return characters is lossier, but is much
  simpler than incrementally maintaining the invariant that
  there are no carriage returns followed by newlines. We may
  want to do something smarter in the future.

Co-authored-by: Keith Simmons <keith@zed.dev>

crates/language/src/tests.rs

@@ -421,7 +421,7 @@ async fn test_outline(cx: &mut gpui::TestAppContext) {
     async fn search<'a>(
         outline: &'a Outline<Anchor>,
         query: &str,
-        cx: &gpui::TestAppContext,
+        cx: &'a gpui::TestAppContext,
     ) -> Vec<(&'a str, Vec<usize>)> {
         let matches = cx
             .read(|cx| outline.search(query, cx.background().clone()))