fix(aio): clean up non-public previews

The previous clean-up code for PR directories on the preview server assumed that
all directories were named after the PR number. With the changes introduced
in #17640 it is possible to have PR directories that do not follow that naming
convention (e.g. "non-public" directories).

This PR ensures that both public and non-public directories are removed when
cleaning up.

aio/aio-builds-setup/dockerbuild/scripts-js/lib/common/constants.ts

@@ -0,0 +1,2 @@
+// Constants
+export const HIDDEN_DIR_PREFIX = 'hidden--';