shell: enhance help command
shell: enhance help command

To get a list of all supported shell commands user must hit the TAB
button. It may be awkward on some terminal emulators. This change
enhances the help command in a way it will in addition print all
supported commands.

Fixes: #28785

Signed-off-by: Jakub Rzeszutko <db01fea5b4491dba4fc272e7096cf665c7b907b9@nordisemi.no>

zephyrproject-rtos/zephyr,Vudentz/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,Vudentz/zephyr,galak/zephyr,Vudentz/zephyr,Vudentz/zephyr,nashif/zephyr,nashif/zephyr,galak/zephyr,finikorg/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,nashif/zephyr,galak/zephyr,finikorg/zephyr,finikorg/zephyr,nashif/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,Vudentz/zephyr