Add zi zsh plugin manager

[b1gb4by]

• add the required functionality for zi

Standards checklist:

• [x] The PR title is descriptive.
• [x] The code compiles (cargo build)
• [x] The code passes rustfmt (cargo fmt)
• [x] The code passes clippy (cargo clippy)
• [x] The code passes tests (cargo test)
• [ ] Optional: I have tested the code myself
  • [ ] I also tested that Topgrade skips the step where needed

If you developed a feature or a bug fix for someone else and you do not have the means to test it, please tag this person here.

[svenXY]

Hi, this breaks non-zi setups that have ${ZPFX} set. It is generally not a good idea to make use of ZPFX variable to decide on certain zsh plugin managers: As https://zdharma-continuum.github.io/zinit/wiki/zsh-plugin-standard/#5_global_parameter_with_prefix_for_make_configure_etc suggests, many plugin managers may use this, in my case, it's zgenom and your code just fails with

zi - command not found

I had already opened a bug report for erroneously using $ZPFX for another plugin manager before - see #862

Would it be possible to use a different way of deciding that zi is used? Maybe just the existence of the .zi directory?

Thanks, Sven

[MCOfficer]

Alternative ways to detect zi's presence are currently discussed in #935