shell: Shell subsystem reimplementation

New shell support features like:
- multi-instance
- command tree
- static and dynamic commands
- multiline
- help print function
- smart tab (autocompletion)
- meta-keys
- history, wildcards etc.
- generic transport (initially, uart present)

Signed-off-by: Jakub Rzeszutko <jakub.rzeszutko@nordicsemi.no>
Signed-off-by: Krzysztof Chruscinski <krzysztof.chruscinski@nordicsemi.no>
Signed-off-by: Piotr Zięcik <piotr.ziecik@nordicsemi.no>