feat: add Carrd, SpaceHey, and Substack as supported sites

- Carrd: Simple website builder with profiles at {username}.carrd.co.
  Uses status_code detection (404 for non-existing profiles).

- SpaceHey: Retro social network inspired by MySpace.
  Uses message detection ("Not Found (Error 404) | SpaceHey" title
  for non-existing profiles).

- Substack: Newsletter/publishing platform with profiles at
  {username}.substack.com. Uses status_code detection (404 for
  non-existing publications).