Support subcommand flags in native completion engine #5284
Description
#3656 added subcommand support but left out pacman-like flag subcommands.
One catch is that subcommand short flags can be conjoined with each other and regular short flags (-Sq). If that is a hassle, we can split that out into a separate issue.
We shouldn't offer hidden options by default but complete them if the user starts typing them
See #3166 for more details on native completions