[WIP] outroing update flow

[pushkine]

Resolves #4683

This is a followup to #4634
while a dirty check in simple if_blocks managed to fix the issue for many, every other update between outrostart and outroend will still either be voided, breaking or worked around using nasty checks such as !current || dirty

Here is an edge case among others :
https://svelte.dev/repl/b71f70fa92a94bd8b82c677b919083a9?version=3.20.1
Fixes #4696, Fixes #4064

• a compound if_block is nested in a simple element that has an outro
• that element is nested in a simple if_block that switches off then on again before outroend
• in the same exact tick it also changes the condition for the nested compound

on first tick a hard outro is triggered on the element, a soft one on the current_block
on second thick, the update goes first, it calls a hard outro on the current_block
since it has not yet done its soft outro, the hard outro is voided
then it calls transition_in, but current_index has changed due to the update
so while the current_block gets transition_in called twice, the previous_block ends its soft outro and stays in the dom

this cannot be fixed in a single line as it necessitates to call transition_in before updating, that comes with the price of breaking every !current || dirty check, so it's a WIP

[pushkine]

#4693 made me realize that it's not just the outro part that needs "edge case" fixing, fragments in general require significant rework
Closing this as I do not plan to address the mentioned issues directly,
I'll submit another pr in a few days