fix(cli): allow `omz` commands to be used in a script (#10645)

The commands `omz plugin {enable,disable}` and `omz theme set`
automatically reload the zsh session on success. With this
change, the CLI checks whether the commands are run in an
interactive session before reloading the zsh session.

This change also conditionally sets the completion function
for `omz` so that it's not done in a non-interactive session.