Refactor dotfiles to work for arbitrary commands

[texastoland]

I attempted to follow your coding style as closely as possible (not stacked anymore) 🕊️

Non-breaking changes

• gm cfg
  • Optionally accepts a closure to work with other Git clients besides git (including gm itself)
  • Removes $env.DOTFILES_WORKTREE because it's redundant between $env.HOME and --work-tree
• gm cfg edit
  • Resolves editor following conventions:
    1. Explicit override (gm cfg edit code)
    2. $env.VISUAL
    3. $env.EDITOR
    4. nano (user-friendly default if nothing else configured)
  • Calls gm cfg internally
• Both add flags for overriding Git vars
• Sanity tested locally (no automated tests)

[texastoland]

Moved to new package at https://github.com/texastoland/dotfiles/blob/main/.config/nushell/scripts/nutex/dot.nu.