shell: Add cpp-friendly shell macro for subcommands creation

Add macro for creating subcommand set in a way that is
accepted by C++ (SHELL_STATIC_SUBCMD_SET_CREATE). Currently,
it exists along with SHELL_CREATE_STATIC_SUBCMD_SET which is
used in the tree but it is not liked by cpp.

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