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_whitespace/src/lib.rs

@@ -0,0 +1,5 @@
+mod newlines;
+mod whitespace;
+
+pub use newlines::*;
+pub use whitespace::*;