Add support for recognizing .ksh scripts and Arch Linux PKGBUILDs

This just changes the language filetypes to include .ksh, and Arch Linux's `PKGBUILD` files, which are used by `makepkg` for creating `pacman` packages.
In addition, it includes changing the name of the language to just be "Shell Script",
to remove possible confusion that this language only works for `bash` shell scripts