Add load_glob directive

[adrusi]

It's generally useful and a widespread practice to allow including a file glob in config formats.

My own use case is to make skhd play nice with GNU Stow dotfile management, where dotfiles are split into different "packages" which are then merged together into the home directory. Without .load_glob, either only one Stow package could include skhd configuration, or the skhd Stow package would have to be modified to reference every Stow package that configures skhd.

The commit adds a new .load_glob directive instead of extending the behavior of .load for compatibility reasons. See kitty's include and globinclude config directives for prior art.