Add CI

[john-kurkowski]

• Add ~/.bin/dotfiles-test test command
  • To avoid polluting the home directory in the repo's designed bare checkout, this PR does not define a package.json or lockfile
  • Run safely in a disposable, non-bare clone, by default
  • Flags:
    • --here to run in the current checkout (so a little less safely)
    • --yes to do this without confirmation
    • --fix to apply available fixes
  • Cover:
    • Markdown formatting with Prettier
    • Lua formatting and lint for the Neovim config
    • Zsh syntax checks
    • Headless Neovim startup smoke test
• Apply fixes
• Configure GitHub Actions
• Remove unused Remark
  • Prettier does most of what I stylistic enforcement I originally needed Remark for