Add nanoseconds to timestamps stub file (#50374)

* Add nanoseconds to timestamps stub file

I also found the documentation to be lacking. I had to try it out to determine if `nanoseconds` would be giving me all nanoseconds until the next full second or just until the next full microsecond (the latter is the case).

* Add fix to `whatsnew`

* Revert "Add fix to `whatsnew`"

This reverts commit 00ae238.

Author: robsdedude

pandas/_libs/tslibs/timestamps.pyi

@@ -70,6 +70,8 @@ class Timestamp(datetime):
     @property
     def microsecond(self) -> int: ...
     @property
+    def nanosecond(self) -> int: ...
+    @property
     def tzinfo(self) -> _tzinfo | None: ...
     @property
     def tz(self) -> _tzinfo | None: ...