[formatter] Fix missing blank lines before decorated classes in .pyi files (#18888)

Co-authored-by: Micha Reiser <micha@reiser.io>

crates/ruff_python_formatter/src/preview.rs

@@ -20,3 +20,10 @@ pub(crate) const fn is_no_chaperone_for_escaped_quote_in_triple_quoted_docstring
 ) -> bool {
     context.is_preview()
 }
+
+/// Returns `true` if the [`blank_line_before_decorated_class_in_stub`](https://github.com/astral-sh/ruff/issues/18865) preview style is enabled.
+pub(crate) const fn is_blank_line_before_decorated_class_in_stub_enabled(
+    context: &PyFormatContext,
+) -> bool {
+    context.is_preview()
+}