Feat: allow extracting multiple children names into one field

[CAD97]

e.g. I have

#[derive(knuffel::Decode)]
pub struct Config {
    #[knuffel(children(name = "replace"))]
    pub replace: Vec<ReplaceRegex>,
    #[knuffel(children(name = "job"))]
    pub jobs: Vec<Job>,
}

It turns out that a lot of replace rules end up being used in practice, and I'd like to allow the use of r as a shorthand. With current knuffel, I'd have to do this as a separate field. I would instead like to do something like the following:

#[derive(knuffel::Decode)]
pub struct Config {
    #[knuffel(children(name = "replace", name = "r"))]
    pub replace: Vec<ReplaceRegex>,
    #[knuffel(children(name = "job"))]
    pub jobs: Vec<Job>,
}

and both replace and r children will go into self.replace.

[tailhook]

Hm, this makes sense. And can also provide another way to achieve #7.