Condense Linux and macOS and support BSD, headless Linux

[Slackadays]

This issue is similar to sindresorhus/clipboardy#92. The project Clipboard supports interfacing with X11, Wayland, Windows, and macOS, so you could replace xclip/sel and wl-clipboard and pbcopy/paste with just clipboard. Additionally, you could also replace Windows, but this doesn't look to already use any command invocations. This change would also add support to the BSDs and headless systems where neither X11 nor Wayland is present.