Unify shell setup on zsh

What does this MR do?

The default shell on MacOS is zsh. In !60 (merged) I merged a user set up with zsh and a user setup with bash and elected to go for bash temporarily in order to simplify that MR. This MR finishes the move and sets us up with zsh.

Set up only the .zshrc since it is loaded both in login and non-login sessions, unlike .bashrc which is only for login sessions and needed .bash_profile also

bash-completion was installed but never actually configured to work. Skipping zsh-completion based on that.

asdf galaxy role used to load the bash completion on zsh. Do our own .zshrc instead.

Why was this MR needed?

What's the best way to test this MR?

What are the relevant issue numbers?

Closes #56 (closed)