Generate shell completions

[alerque]

I just started playing around with updating the Arch Linux packaging in preparation for the v3 release.

Given that the Rust rewrite is using Clap, it should be pretty trivial to generate shell completions for Bash, Zsh, Fish, Elvish, and Powershell should be pretty trivial using clap_completions. This is a nice user convenience to have and makes the most sense in the context of distro packages where they would get automatically installed to the right places automatically.

There is not reason this has to make the final v3 cut, but I figured whether early or late a tracking issue is warranted so downstream packages know if it is or isn't available and how it is generated.

[ku1ik]

As I also mentioned in #627 I plan to add this soon 👍

I see clap_complete supports generation both at compile-time and run-time, so we have options here as wells. Initially I thought runtime generation would be most useful, especially for people who download standalone precompiled binaries. Runtime generation can also be used to generate completion files for packages (by redirecting the output to completion files in proper paths within a package dir tree). Although, we could have both, doesn't seem too hard.

Thoughts?

[alerque]

Again, personally I much prefer build-time for this, but both ways have pros and cons.

[ku1ik]

You can now generate shell completions during build by setting ASCIINEMA_GEN_DIR env var - the files will be saved in $ASCIINEMA_GEN_DIR/completion/.