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_ast/src/lib.rs

@@ -2,6 +2,7 @@ pub mod all;
 pub mod call_path;
 pub mod cast;
 pub mod comparable;
+pub mod docstrings;
 pub mod function;
 pub mod hashable;
 pub mod helpers;