shell: fix tab for dynamic commands

The tabulator handler creates a single structure if it is handling
dynamic commands. If the currently processed dynamic command has a
dynamic subcommand they both share the same structure.
As a result tabulation operation may result in undefined behaviour.

As a solution, a new structure was introduced to keep subcommand
information.

Fixes #35926.

Signed-off-by: Jakub Rzeszutko <jakub.rzeszutko@nordicsemi.no>