editor: Improve find_matches and replace_all perf (#51941)

Helps with https://github.com/zed-industries/zed/issues/38927

- **editor: Add a benchmark for find/replace**
- **text: batch fragment insertions before turning them into a SumTree**

## Context

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## Self-Review Checklist

<!-- Check before requesting review: -->
- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [ ] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Release Notes:

- Improved performance of "Replace All" in buffer search

---------

Co-authored-by: Smit Barmase <heysmitbarmase@gmail.com>

crates/multi_buffer/src/multi_buffer.rs

@@ -5258,6 +5258,16 @@ impl MultiBufferSnapshot {
         Some(Anchor::in_buffer(path_key_index, anchor))
     }
 
+    /// Lifts a buffer anchor range to a multibuffer anchor range without checking against excerpt boundaries. Returns `None` if there are no excerpts for the buffer.
+    pub fn anchor_range_in_buffer(&self, range: Range<text::Anchor>) -> Option<Range<Anchor>> {
+        if range.start.buffer_id != range.end.buffer_id {
+            return None;
+        }
+
+        let path_key_index = self.path_key_index_for_buffer(range.start.buffer_id)?;
+        Some(Anchor::range_in_buffer(path_key_index, range))
+    }
+
     /// Creates a multibuffer anchor for the given buffer anchor, if it is contained in any excerpt.
     pub fn anchor_in_excerpt(&self, text_anchor: text::Anchor) -> Option<Anchor> {
         let excerpts = {
@@ -5295,6 +5305,19 @@ impl MultiBufferSnapshot {
         &self,
         text_anchor: Range<text::Anchor>,
     ) -> Option<Range<Anchor>> {
+        if self.is_singleton() {
+            let excerpt = self.excerpts.first()?;
+            let buffer_snapshot = excerpt.buffer_snapshot(self);
+            if excerpt.range.contains(&text_anchor.start, &buffer_snapshot)
+                && excerpt.range.contains(&text_anchor.end, &buffer_snapshot)
+            {
+                return Some(Anchor::range_in_buffer(excerpt.path_key_index, text_anchor));
+            }
+        }
+
+        // for each search match
+
+        let mut buffer_snapshot = None;
         for excerpt in {
             let this = &self;
             let buffer_id = text_anchor.start.buffer_id;
@@ -5316,7 +5339,8 @@ impl MultiBufferSnapshot {
             .into_iter()
             .flatten()
         } {
-            let buffer_snapshot = excerpt.buffer_snapshot(self);
+            let buffer_snapshot =
+                buffer_snapshot.get_or_insert_with(|| excerpt.buffer_snapshot(self));
             if excerpt.range.contains(&text_anchor.start, &buffer_snapshot)
                 && excerpt.range.contains(&text_anchor.end, &buffer_snapshot)
             {