Use Cursor for shebang parsing (#5716)

## Summary

Better to leverage the shared functionality we get from `Cursor`. It's
also a little bit faster, which is very cool.

crates/ruff/src/comments/shebang.rs

@@ -0,0 +1,67 @@
+use ruff_python_whitespace::{is_python_whitespace, Cursor};
+use ruff_text_size::{TextLen, TextSize};
+
+/// A shebang directive (e.g., `#!/usr/bin/env python3`).
+#[derive(Debug, PartialEq, Eq)]
+pub(crate) struct ShebangDirective<'a> {
+    /// The offset of the directive contents (e.g., `/usr/bin/env python3`) from the start of the
+    /// line.
+    pub(crate) offset: TextSize,
+    /// The contents of the directive (e.g., `"/usr/bin/env python3"`).
+    pub(crate) contents: &'a str,
+}
+
+impl<'a> ShebangDirective<'a> {
+    /// Parse a shebang directive from a line, or return `None` if the line does not contain a
+    /// shebang directive.
+    pub(crate) fn try_extract(line: &'a str) -> Option<Self> {
+        let mut cursor = Cursor::new(line);
+
+        // Trim whitespace.
+        cursor.eat_while(is_python_whitespace);
+
+        // Trim the `#!` prefix.
+        if !cursor.eat_char('#') {
+            return None;
+        }
+        if !cursor.eat_char('!') {
+            return None;
+        }
+
+        Some(Self {
+            offset: line.text_len() - cursor.text_len(),
+            contents: cursor.chars().as_str(),
+        })
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use insta::assert_debug_snapshot;
+
+    use super::ShebangDirective;
+
+    #[test]
+    fn shebang_non_match() {
+        let source = "not a match";
+        assert_debug_snapshot!(ShebangDirective::try_extract(source));
+    }
+
+    #[test]
+    fn shebang_end_of_line() {
+        let source = "print('test')  #!/usr/bin/python";
+        assert_debug_snapshot!(ShebangDirective::try_extract(source));
+    }
+
+    #[test]
+    fn shebang_match() {
+        let source = "#!/usr/bin/env python";
+        assert_debug_snapshot!(ShebangDirective::try_extract(source));
+    }
+
+    #[test]
+    fn shebang_leading_space() {
+        let source = "  #!/usr/bin/env python";
+        assert_debug_snapshot!(ShebangDirective::try_extract(source));
+    }
+}