Pass 'intro/outro' parameter to transition generators?

[mrkishi]

Currently, we can't find out if we are introing or outroing from a "transition generator" (that's what I'm calling the functions that are returned from transition functions -- the ones that are called just before a transition starts to get a TransitionConfig object).

While we can use separate in: and out: directives (like for crossfade), for a number of use cases that's unneeded boilerplate.

I propose that we start passing an intro: boolean to them:

export function my_transition(node: Element, params: any) {
    return function (intro: boolean) {
        return intro ? wacky_intro_params(params) : coordinated_outro(params);
    };
}

This wouldn't be a breaking change, but it would expand on the contract we have to support in the future. Personally, I think it's worth it, being such a small change, but what are our thoughts on it?

I tried coming up with hacky ways to figure out if it's an intro but came up with nothing that worked reliably.

[Conduitry]

Is this the same as #3918?

[mrkishi]

Ugh, I don't know how I missed that one.

It's not quite the same, but indeed very similar--I'll post there instead.