Make script POSIX compliant

[delvh]

Currently, this script can only be executed on bash and not on other shells.
For example, when I on Ubuntu 20.04 call

sudo sh energized.sh

everything looks horrible (i.e. the -e of echo will be printed for each command), and the program cannot be used at all (when I try to select any of the packs, it always returns instead of accepting, even for valid inputs), sometimes the shell even complains that it does not know what [[ should be.
This is because Ubuntu does not have bash as default anymore (/usr/bin/sh is a symbolic link to dash).
It would be much more user friendly if this script would not only work on bash, but on any shell instead.
This can be achieved by making it POSIX compliant.