Rewrite placement logic (#6040)

## Summary
This is a rewrite of the main comment placement logic. `place_comment`
now has three parts:

- place own line comments
  - between branches
  - after a branch
- place end-of-line comments
  - after colon
  - after a branch
- place comments for specific nodes (that include module level comments)

The rewrite fixed three bugs: `class A: # trailing comment` comments now
stay end-of-line, `try: # comment` remains end-of-line and deeply
indented try-else-finally comments remain with the right nested
statement.

It will be much easier to give more alternative branches nodes since
this is abstracted away by `is_node_with_body` and the first/last child
helpers. Adding new node types can now be done by adding an entry to the
`place_comment` match. The code went from 1526 lines before #6033 to
1213 lines now.

It thinks it easier to just read the new `placement.rs` rather than
reviewing the diff.

## Test Plan

The existing fixtures staying the same or improving plus new ones for
the bug fixes.

crates/ruff_python_formatter/src/expression/expr_slice.rs

@@ -132,14 +132,12 @@ impl FormatNodeRule<ExprSlice> for FormatExprSlice {
                 let step_leading_comments = comments.leading_comments(step.as_ref());
                 leading_comments_spacing(f, step_leading_comments)?;
                 step.format().fmt(f)?;
-            } else {
-                if !dangling_step_comments.is_empty() {
-                    // Put the colon and comments on their own lines
-                    write!(
-                        f,
-                        [hard_line_break(), dangling_comments(dangling_step_comments)]
-                    )?;
-                }
+            } else if !dangling_step_comments.is_empty() {
+                // Put the colon and comments on their own lines
+                write!(
+                    f,
+                    [hard_line_break(), dangling_comments(dangling_step_comments)]
+                )?;
             }
         } else {
             debug_assert!(step.is_none(), "step can't exist without a second colon");