We should expose completion-related commands as regular CLI commands so that it's easier for users to look them up and use them.

Currently we have a number of completion-related commands (see https://pnp.github.io/office365-cli/concepts/completion/ for more info). These commands are handled internally and other than through using the CLI docs, there is no way for users to find out that they're there. To make things easier and consistent, these commands should be treated as regular commands exposed under the cli namespace.