Add zgenom zsh plugin manager

[svenXY]

• add the required functionality for zgenom
• fix the env::var for zinit - it should be ZINIT_HOME, not ZBXF, as this is also set in zgenom, but makes topgrade fail when zinit is not installed

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) - with some warnings that have nothin to do with my code
• [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.