Compare formatted and unformatted ASTs during formatter tests (#8624)

## Summary

This PR implements validation in the formatter tests to ensure that we
don't modify the AST during formatting. Black has similar logic.

In implementing this, I learned that Black actually _does_ modify the
AST, and their test infrastructure normalizes the AST to wipe away those
differences. Specifically, Black changes the indentation of docstrings,
which _does_ modify the AST; and it also inserts parentheses in `del`
statements, which changes the AST too.

Ruff also does both these things, so we _also_ implement the same
normalization using a new visitor that allows for modifying the AST.

Closes https://github.com/astral-sh/ruff/issues/8184.

## Test Plan

`cargo test`

crates/ruff_python_ast/src/visitor.rs

@@ -1,6 +1,7 @@
 //! AST visitor trait and walk functions.
 
 pub mod preorder;
+pub mod transformer;
 
 use crate::{
     self as ast, Alias, Arguments, BoolOp, CmpOp, Comprehension, Decorator, ElifElseClause,
@@ -14,8 +15,10 @@ use crate::{
 /// Prefer [`crate::statement_visitor::StatementVisitor`] for visitors that only need to visit
 /// statements.
 ///
-/// Use the [`PreorderVisitor`](self::preorder::PreorderVisitor) if you want to visit the nodes
+/// Use the [`PreorderVisitor`](preorder::PreorderVisitor) if you want to visit the nodes
 /// in pre-order rather than evaluation order.
+///
+/// Use the [`Transformer`](transformer::Transformer) if you want to modify the nodes.
 pub trait Visitor<'a> {
     fn visit_stmt(&mut self, stmt: &'a Stmt) {
         walk_stmt(self, stmt);