Skip over folded regions when iterating over multibuffer chunks (#15646)

This commit weaves through new APIs for language::BufferChunks, multi_buffer::MultiBufferChunks and inlay_map::InlayChunks that allow seeking with an upper-bound. This allows us to omit doing syntax highligting and looking up diagnostics for folded ranges. This in turn directly improves performance of assistant panel with large contexts.

Release Notes:

- Fixed poor performance when editing in the assistant panel after
inserting large files using slash commands

---------

Co-authored-by: Max <max@zed.dev>
Co-authored-by: Max Brunsfeld <maxbrunsfeld@gmail.com>

crates/editor/src/editor_tests.rs

@@ -3668,6 +3668,63 @@ fn test_duplicate_line(cx: &mut TestAppContext) {
         );
     });
 }
+#[gpui::test]
+async fn test_fold_perf(cx: &mut TestAppContext) {
+    use std::fmt::Write;
+    init_test(cx, |_| {});
+    let mut view = EditorTestContext::new(cx).await;
+    let language_registry = view.language_registry();
+    let language_name = Arc::from("Markdown");
+    let md_language = Arc::new(
+        Language::new(
+            LanguageConfig {
+                name: Arc::clone(&language_name),
+                matcher: LanguageMatcher {
+                    path_suffixes: vec!["md".to_string()],
+                    ..Default::default()
+                },
+                ..Default::default()
+            },
+            Some(tree_sitter_md::language()),
+        )
+        .with_highlights_query(
+            r#"
+        "#,
+        )
+        .unwrap(),
+    );
+    language_registry.add(md_language.clone());
+
+    let mut text = String::default();
+    writeln!(&mut text, "start").unwrap();
+    writeln!(&mut text, "```").unwrap();
+    const LINE_COUNT: u32 = 10000;
+    for i in 0..LINE_COUNT {
+        writeln!(&mut text, "{i}").unwrap();
+    }
+
+    writeln!(&mut text, "```").unwrap();
+    writeln!(&mut text, "end").unwrap();
+    view.update_buffer(|buffer, cx| {
+        buffer.set_language(Some(md_language), cx);
+    });
+    let t0 = Instant::now();
+    _ = view.update_editor(|view, cx| {
+        eprintln!("Text length: {}", text.len());
+        view.set_text(text, cx);
+        eprintln!(">>");
+        view.fold_ranges(
+            vec![(
+                Point::new(1, 0)..Point::new(LINE_COUNT + 2, 3),
+                FoldPlaceholder::test(),
+            )],
+            false,
+            cx,
+        );
+    });
+    eprintln!("{:?}", t0.elapsed());
+    eprintln!("<<");
+}
 
 #[gpui::test]
 fn test_move_line_up_down(cx: &mut TestAppContext) {