shell: Extract shell creation from SHELL_DEFINE

Extract shell instance creation from SHELL_DEFINE to Z_SHELL_DEFINE.
SHELL_DEFINE creates logging backend and calls Z_SHELL_DEFINE to
create a shell instance with that logging backend. With this split
it is possible to create a shell instance with logging integration
but without the default shell logging backend.

Signed-off-by: Krzysztof Chruściński <krzysztof.chruscinski@nordicsemi.no>