Add completion subcommand

[tranzystorekk]

There seem to have been a few attempts at completions in the past, but generating from a subcommand seems to be the most versatile for both users and package maintainers. Maybe the old manually written completion scripts could be ditched as well for simplicity?

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[Keats]

Honestly I'm kind of wondering if it's even worth adding considering how few commands zola has.

[tranzystorekk]

IMO even two commands is a good reason to have them, and zola serve in particular has quite a few flags that also get completed.

[c-git]

If I may be permitted to add my opinion. It is always wonderful if a program supports completions. It's not required but a pleasant surprise if you find out it's available. If there are no material downsides I would say this is a nice thing to add. Are there any notable disadvantages to having this added? I did note that some things needed to be commented out but I didn't quite understand what they were.

PS. I've really come to appreciate how careful @Keats is with regard to what is added. I can't tell for sure but it makes it seem like you have a vision for the project and you try to keep it on that vision while being open to other people's ideas. You never just shoot things down without a reason. Even like this you gave your reasoning why you are reluctant. Kudos to you.

[Keats]

I'll try locally later to see how it works in practice

[tranzystorekk]

If we merge this, I'd point it out in the changelog that the scripts in completion/ are obsolete so the package maintainers know what to use, or even remove that whole dir in this PR

[Keats]

Let's remove them in that PR, they haven't been updated in years anyway.

[Keats]

Thanks!