fix: properly delay intro transitions

[dummdidumm]

WAAPI applies the styles of a delayed animation only when that animation starts. In the case of fade-in transitions that means the element is visible, then goes invisible and fades in. Fix that by never applying a delay on intro transitions, instead add keyframes of the initial state for the duration of the delay.

Fixes #10876

This works, but the test is failing - I can't figure out how to properly write one. For some reason the opacity that the fade transition calculates is 0, whereas it should be 1 (and in fact is, when testing this in the playground). Update: got it working by applying the style manually, but feels like a hack and it doesn't work for the outro transition, still no idea why this happens

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[trueadm]

What about outro transitions? If it starts hidden in the outro, then fades in then out again, does that work correctly with delay?

[dummdidumm]

That's different because the delay says "the animation starts after X miliseconds", and so I would expect the invisible->visible->invisible transition to start only after the given delay. For intro transitions it makes sense to apply the initial value directly because the transitions all start from an invisible state and then go to a visible one.
That said, maybe someone has a bonkers intro transitions where they start in a visible state, meaning the element shows up directly. I'm not sure what the expectation here is then: Should it apply the initial value for the duration of the delay, i.e. be visible, or should it invisible until it starts (because you're starting from "element doesn't exist")? If we say the latter, we could hard-code opacitiy: 0 for the duration of the delay.
I just checked what Svelte 4 does, and applies the same logic as this PR: immediately apply first frame on intro, delay on outro.

[Rich-Harris]

oops i approved but there's a lint error, looking