perf: Dont export stable in narwhals/__init__.py, instead require import narwhals.stable (#2480)

Co-authored-by: Dan Redding <[email protected]>

Author: MarcoGorelli

narwhals/__init__.py

@@ -4,7 +4,6 @@
 from narwhals import dtypes
 from narwhals import exceptions
 from narwhals import selectors
-from narwhals import stable
 from narwhals.dataframe import DataFrame
 from narwhals.dataframe import LazyFrame
 from narwhals.dtypes import Array
@@ -160,7 +159,6 @@
     "scan_parquet",
     "selectors",
     "show_versions",
-    "stable",
     "sum",
     "sum_horizontal",
     "to_native",