Enhance match to include multiple targets

[PizzasBear]

This adds ability to or patterns in any context (e.g. in when and let).
This is done by adding an OrChain variant to the Target enum in askama_parser,
and simply adding a | between or-chained patterns in askama_derive.

Closes #904

[PizzasBear]

I've removed the comments, forgot to remove them prior to committing...

[djc]

I don't find this <[_; 1]>::try_from() idiom particularly clear. How about this:

match opts.len() {
    1 => opts.pop().unwrap(),
    _ => Self::OrChain(opts),
}

(Is this actually necessary? What happens if we just always yield a Vec<Target<'a>> from parse()?)

[PizzasBear]

I liked the try_from because it avoided mut and .unwrap(), but I can change this.
The match statement helps avoid one item Target::OrChain which will help debugging by cleaning up the output of println!("{target:#?}") or other statements that'll result in pretty printing a target.