Emacs: my new 'sxhkdrc-mode' package

This is a major mode for editing sxhkdrc files. SXHKD is the Simple X Hot Key Daemon which is commonly used in minimalist desktop sessions on Xorg, such as with the Binary Space Partitioning Window Manager (BSPWM). The sxhkdrc file configures key chords, binding them to commands. For the technicalities, read the man page sxhkd(1).

This version of sxhkdrc-mode defines fontification rules, a way to indent keys and commands, and sets up the file as a derivative of prog-mode. In future versions, we will consider adding support for more features, depending on the needs of the community.