wifi: Refactor wifi-shell to allow external subcommands

Rework the wifi-shell code so that external subcommands can be
bolted into it easily. This means that the commands should be
created using SHELL_SUBCMD_ADD().

Place the various subcommands in alphabetical order to find them
more easily.

Split long lines in order to pass CI checks.

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>