[ENH] add exports of common utilities in utils module (#5266)

This PR adds exports of common utilities in `utils` module.

The selection aims at those that users or developers would frequently
import.

sktime/utils/__init__.py

@@ -1 +1,10 @@
 """Utility functionality."""
+
+from sktime.utils.estimator_checks import check_estimator
+from sktime.utils.plotting import plot_series, plot_windows
+
+__all__ = [
+    "check_estimator",
+    "plot_series",
+    "plot_windows",
+]