Patch Changes

• 97d5fb2: Add external links validation via the flag --validate-externals.

  You can/should provide an optional --absolute-base-url to handle urls starting with it as internal.

  # check external urls
  npx check-html-links _site --validate-externals
  
  # check external urls but treat links like https://rocket.modern-web.dev/about/ as internal
  npx check-html-links _site --validate-externals --absolute-base-url https://rocket.modern-web.dev