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/whitespace.rs

@@ -0,0 +1,15 @@
+/// Returns `true` for [whitespace](https://docs.python.org/3/reference/lexical_analysis.html#whitespace-between-tokens)
+/// characters.
+pub const fn is_python_whitespace(c: char) -> bool {
+    matches!(
+        c,
+        // Space, tab, or form-feed
+        ' ' | '\t' | '\x0C'
+    )
+}
+
+/// Extract the leading indentation from a line.
+pub fn leading_indentation(line: &str) -> &str {
+    line.find(|char: char| !is_python_whitespace(char))
+        .map_or(line, |index| &line[..index])
+}