Make paste a separate undo transaction from preceding edits (#52003)

When undoing a paste, it is really confusing when that actually also
removes what was type right before the paste if the paste happened fast
enough after.

Release Notes:

- Fixed undoing a paste sometimes also undoing edits right before the
paste

crates/multi_buffer/src/multi_buffer_tests.rs

@@ -3513,8 +3513,8 @@ fn test_history(cx: &mut App) {
         buf
     });
     let multibuffer = cx.new(|_| MultiBuffer::new(Capability::ReadWrite));
-    multibuffer.update(cx, |this, _| {
-        this.set_group_interval(group_interval);
+    multibuffer.update(cx, |this, cx| {
+        this.set_group_interval(group_interval, cx);
     });
     multibuffer.update(cx, |multibuffer, cx| {
         multibuffer.set_excerpts_for_path(