shell-completions/bash: Add pactl set-default-sink and set-default-source

shell-completions/bash: Add pactl set-default-sink and set-default-source

pactl has these commands, too. Use the same order as the
man page, except the undocumented 'help'. Note that the commands are
sorted alphabetically when completed anyway, though
that can be disabled since Bash 4.4
(https://unix.stackexchange.com/q/215937/111181)

The zsh completions already support set-default-sink and
set-default-source.