Move Python whitespace utilities into new ruff_python_whitespace crate (#4993)

## Summary

`ruff_newlines` becomes `ruff_python_whitespace`, and includes the
existing "universal newline" handlers alongside the Python
whitespace-specific utilities.

crates/ruff_python_formatter/src/comments/placement.rs

@@ -1,10 +1,10 @@
 use crate::comments::visitor::{CommentPlacement, DecoratedComment};
 use crate::comments::CommentTextPosition;
 use crate::trivia::{SimpleTokenizer, TokenKind};
-use ruff_newlines::StrExt;
 use ruff_python_ast::node::AnyNodeRef;
 use ruff_python_ast::source_code::Locator;
 use ruff_python_ast::whitespace;
+use ruff_python_whitespace::UniversalNewlines;
 use ruff_text_size::{TextRange, TextSize};
 use rustpython_parser::ast::Ranged;
 use std::cmp::Ordering;

crates/ruff_python_formatter/src/comments/visitor.rs

@@ -9,7 +9,7 @@ use ruff_python_ast::source_code::{CommentRanges, Locator};
 // pre-order.
 #[allow(clippy::wildcard_imports)]
 use ruff_python_ast::visitor::preorder::*;
-use ruff_python_ast::whitespace::is_python_whitespace;
+use ruff_python_whitespace::is_python_whitespace;
 use ruff_text_size::TextRange;
 use std::iter::Peekable;