[flake8-simplify] Implement enumerate-for-loop (SIM113) (#7777)

Implements SIM113 from #998

Added tests
Limitations 
   - No fix yet
   - Only flag cases where index variable immediately precede `for` loop

@charliermarsh please review and let me know any improvements

---------

Co-authored-by: Charlie Marsh <charlie.r.marsh@gmail.com>

crates/ruff_python_ast/src/traversal.rs

@@ -79,3 +79,15 @@ pub fn next_sibling<'a>(stmt: &'a Stmt, suite: &'a Suite) -> Option<&'a Stmt> {
     }
     None
 }
+
+/// Given a [`Stmt`] and its containing [`Suite`], return the previous [`Stmt`] in the [`Suite`].
+pub fn prev_sibling<'a>(stmt: &'a Stmt, suite: &'a Suite) -> Option<&'a Stmt> {
+    let mut prev = None;
+    for sibling in suite {
+        if sibling == stmt {
+            return prev;
+        }
+        prev = Some(sibling);
+    }
+    None
+}