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/docstrings.rs

@@ -0,0 +1,22 @@
+//! Utilities for parsing Python docstrings.
+
+/// Extract the leading words from a line of text within a Python docstring.
+pub fn leading_words(line: &str) -> &str {
+    let line = line.trim();
+    line.find(|char: char| !char.is_alphanumeric() && !char.is_whitespace())
+        .map_or(line, |index| &line[..index])
+}
+
+/// Extract the leading whitespace from a line of text within a Python docstring.
+pub fn leading_space(line: &str) -> &str {
+    line.find(|char: char| !char.is_whitespace())
+        .map_or(line, |index| &line[..index])
+}
+
+/// Replace any non-whitespace characters from an indentation string within a Python docstring.
+pub fn clean_space(indentation: &str) -> String {
+    indentation
+        .chars()
+        .map(|char| if char.is_whitespace() { char } else { ' ' })
+        .collect()
+}