feat: ability to set program for zinit ls to use

[vladdoster]

Description

A user can now specify a program via the $ZINIT[LIST_COMMAND] variable.
Additionally, when unset, the default behavior is to first check for exa, then
tree, and ls as a last resort.

e.g., use exa or any tree-esque program in zinit ls.

• Add unit test for tree

Closes #170

Motivation and Context

See #170

Usage examples

Before sourcing zinit.zsh, set $ZINIT[LIST_COMMAND]:

typeset -gAH ZINIT;

# A safe choice is to use `ls`
$ZINIT[LIST_COMMAND]='ls --tree'

command chmod g-rwX $ZINIT[HOME_DIR] \
 && zcompile $ZINIT[BIN_DIR]/zinit.zsh \
 && source $ZINIT[BIN_DIR]/zinit.zsh \
 && autoload -Uz _zinit \
 && (( ${+_comps} )) \
 && _comps[zinit]=_zinit

How Has This Been Tested?

Manually

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to
  change)

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

[github-actions]

🎉 This PR is included in version 3.8.0 🎉

The release is available on GitHub release

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[turboBasic]

@vladdoster where does ls --tree come from? Neither macOS ls, not GNU coreutils ls, nor Ubuntu's ls have --tree option:

/bin/ls --tree                                           # built-in macOS ls
# ls: unrecognized option `--tree'
# usage: ls [-@ABCFGHILOPRSTUWabcdefghiklmnopqrstuvwxy1%,] [--color=when] [-D format] [file ...]


/opt/homebrew/bin/gls --tree                             # ls from Homebrew coreutils package (GNU ls)
# /opt/homebrew/bin/gls: unrecognized option '--tree'
# Try '/opt/homebrew/bin/gls --help' for more information.


docker run --rm --network=host ubuntu:24.04 ls --tree    # ls from official Ubuntu image (GNU ls)
# ls: unrecognized option '--tree'
# Try 'ls --help' for more information.

Man page for GNU ls(1) also does not list --tree option:
https://man7.org/linux/man-pages/man1/ls.1.html