Fix blank-line docstring rules for module-level docstrings (#9878)

## Summary

Given:

```python
"""Make a summary line.

Note:
----
  Per the code comment the next two lines are blank. "// The first blank line is the line containing the closing
      triple quotes, so we need at least two."

"""
```

It turns out we excluded the line ending in `"""`, because it's empty
(unlike for functions, where it consists of the indent). This PR changes
the `following_lines` iterator to always include the trailing newline,
which gives us correct and consistent handling between function and
module-level docstrings.

Closes https://github.com/astral-sh/ruff/issues/9877.

crates/ruff_source_file/src/newlines.rs

@@ -184,11 +184,18 @@ impl<'a> Iterator for NewlineWithTrailingNewline<'a> {
     type Item = Line<'a>;
 
     #[inline]
-    fn next(&mut self) -> Option<Line<'a>> {
+    fn next(&mut self) -> Option<Self::Item> {
         self.underlying.next().or_else(|| self.trailing.take())
     }
 }
 
+impl DoubleEndedIterator for NewlineWithTrailingNewline<'_> {
+    #[inline]
+    fn next_back(&mut self) -> Option<Self::Item> {
+        self.trailing.take().or_else(|| self.underlying.next_back())
+    }
+}
+
 #[derive(Debug, Clone, Eq, PartialEq)]
 pub struct Line<'a> {
     text: &'a str,