feat: Shell completion

### Support shell completion?

click has support for completion [documented here](https://click.palletsprojects.com/en/8.1.x/shell-completion/)

Is there any reason we shouldn't support it?

### Proposal

Was thinking something along the lines of adding the allowing the following in ~/.zshrc, or ~/.bashrc

```sh
source <(sam completion)
```

Where we would detect if the completion is for `zsh` or `bash`, and error if the shell is unsupported.