feat: add Parent() accessor to Command

[happysnaker]

Fixes #2372

Adds a Parent() method to Command that returns the parent command, or nil for the root command. This allows users to check if a parent is hidden when walking the command tree.

Usage

cmd.Walk(func(c *cli.Command) error {
    if c.Hidden || (c.Parent() != nil && c.Parent().Hidden) {
        return nil
    }
    // process visible command
    return nil
})