Combine behavioral struct fields into sub-struct groupings

[meatballhat]

Some of the fields on Command are related and may be grouped together for clarity. For example, these Help.+ fields could be grouped together under a Help field:

• HideHelp ➡️ Help.Hidden
• HideHelpCommand ➡️ Help.CommandHidden

or these fields related to shell completion could be under a ShellCompletion field:

• EnableShellCompletion ➡️ ShellCompletion.Enabled
• ShellCompletionCommandName ➡️ ShellCompletion.CommandName

or all of these could be grouped under an Options or Config field (?):

cmd := &cli.Command{
    Name: "foo",
    Config: cli.CommandConfig{
        HideHelp: true,
        EnableShellCompletion: true,
    },
}

There are arguable tradeoffs, of course, such as:

• nested fields are more difficult to reason about and more visually complex than a single level of fields
• logical groupings of fields "hides" them from each other
• depending on the "zero value" of simple types means that default values for certain behaviors are tightly coupled to the field names, e.g. HideSomething (default on) vs. EnableSomething (default off)
• others I'm surely not thinking about

Ideas and questions and concerns wanted 🙇🏼

[bartekpacia]

I understand this issue, but I think the way it is currently is just... good enough. I also think that the less we break from v2, the better.

"Flat is better than nested." - The Zen of Python