Generate completions

[daurnimator]

trafficstars

Add shell completion support.

pub CompletionShell = enum {bash, fish, zsh};
pub fn writeCompletions(shell: CompletionShell, myoutstream: var) !void`

Bonus feature: add a utility method to add a completion subcommand

[ikskuh]

https://github.com/luarocks/argparse/blob/master/src/argparse.lua#L1240-L1346