Format ExprIfExp (ternary operator) (#5597)

## Summary

Format `ExprIfExp`, also known as the ternary operator or inline `if`.
It can look like
```python
a1 = 1 if True else 2
```
but also
```python
b1 = (
    # We return "a" ...
    "a" # that's our True value
    # ... if this condition matches ...
    if True # that's our test
    # ... otherwise we return "b§
    else "b" # that's our False value
)
```

This also fixes a visitor order bug.

The jaccard index on django goes from 0.911 to 0.915.

## Test Plan

I added fixtures without and with comments in strange places.

crates/ruff_python_formatter/src/expression/expr_if_exp.rs

@@ -1,21 +1,42 @@
-use crate::comments::Comments;
+use crate::comments::{leading_comments, Comments};
 use crate::expression::parentheses::{
     default_expression_needs_parentheses, NeedsParentheses, Parentheses, Parenthesize,
 };
-use crate::{not_yet_implemented_custom_text, FormatNodeRule, PyFormatter};
-use ruff_formatter::{write, Buffer, FormatResult};
+use crate::{AsFormat, FormatNodeRule, PyFormatter};
+use ruff_formatter::prelude::{group, soft_line_break_or_space, space, text};
+use ruff_formatter::{format_args, write, Buffer, FormatResult};
 use rustpython_parser::ast::ExprIfExp;
 
 #[derive(Default)]
 pub struct FormatExprIfExp;
 
 impl FormatNodeRule<ExprIfExp> for FormatExprIfExp {
-    fn fmt_fields(&self, _item: &ExprIfExp, f: &mut PyFormatter) -> FormatResult<()> {
+    fn fmt_fields(&self, item: &ExprIfExp, f: &mut PyFormatter) -> FormatResult<()> {
+        let ExprIfExp {
+            range: _,
+            test,
+            body,
+            orelse,
+        } = item;
+        let comments = f.context().comments().clone();
+        // We place `if test` and `else orelse` on a single line, so the `test` and `orelse` leading
+        // comments go on the line before the `if` or `else` instead of directly ahead `test` or
+        // `orelse`
         write!(
             f,
-            [not_yet_implemented_custom_text(
-                "NOT_IMPLEMENTED_true if NOT_IMPLEMENTED_cond else NOT_IMPLEMENTED_false"
-            )]
+            [group(&format_args![
+                body.format(),
+                soft_line_break_or_space(),
+                leading_comments(comments.leading_comments(test.as_ref())),
+                text("if"),
+                space(),
+                test.format(),
+                soft_line_break_or_space(),
+                leading_comments(comments.leading_comments(orelse.as_ref())),
+                text("else"),
+                space(),
+                orelse.format()
+            ])]
         )
     }
 }