0.9.21

Added

• Mob.Storage.exclude_from_backup/1 and Mob.Storage.excluded_from_backup?/1.
  Apple rejects apps that back up data they can download again, and both
  :documents and :app_support are backed up to iCloud on iOS. The first
  sets NSURLIsExcludedFromBackupKey on a file or directory (a directory's
  flag covers its contents); the second reads it back. On Android, where
  generated apps set android:allowBackup="false", and off device they
  return :ok / false without effect. The Mob.Storage docs and the
  device capabilities guide now say which locations are backed up. No
  MobBridge.kt change, but the iOS half is in ios/mob_nif.m: rebuild
  natively (mix mob.deploy --native). On an iOS build that predates it,
  exclude_from_backup/1 returns {:error, :unsupported} rather than a
  false :ok.