Allow reactivity in outroing elements

[7nik]

Describe the problem

During outro, the reactivity in the transitioning elements stops. I guess it is to prevent errors in a case like the following

{#if user}
  <h1 transition:fly>{user.name}</h1>
{/if

But in some other cases it is desired to have working reactivity.

Describe the proposed solution

Allow to opt out of this guarding.

Importance

would make my life easier

[rChaoz]

Maybe somehow related, bug where this would happen #16148