Support command completion

### Is your feature request related to a problem? Please describe.
(my opinion) Shell command completion provides a more interactive experience for finding the options you want to use, a bit more engaging and helpful than `--help` in certain cases. Lazygit does not provide it by default.

### Describe the solution you'd like
Add a completion generator so that packagers can automatically add it or users can add it themselves when building from source

### Describe alternatives you've considered
Creating a personal completion file or continue to reference `--help`

### Additional context
Flaggy (the cli args parser dependency used for this project) has support for generating completions out of the box, but only on the [latest version](https://github.com/integrii/flaggy/releases/tag/v1.8.0). We can upgrade our dependency to fix this. One [breaking change](https://github.com/integrii/flaggy/releases/tag/v1.6.0) they note is that is the way you specify flags with multiple values, which affects `--use-config-file`. Could just put it in release notes as a breaking change and seems ok to me.

An alternative is to switch to another package, but not sure if there's strong pull for that.