fix: overhaul each blocks (#17150)

* state.items -> state.onscreen

* offscreen_items -> state.offscreen

* WIP

* WIP

* WIP

* WIP

* WIP

* WIP

* WIP

* WIP

* always push

* fix

* note to self

* tweak/fix

* tidy up

* simplify

* unused

* clarity

* note to self

* simplify

* one test to go

* holy shit i think it works

* unused

* reduce number of bitwise checks

* revert

* small tweak

* better comment

* update note to self

* fix: each block losing reactivity when items removed while promise pending (#17151)

Co-authored-by: dylan <dylanbradshaw107@hotmail.com>
Co-authored-by: Rich Harris <rich.harris@vercel.com>

* add commented-out test, todo investigate

---------

Co-authored-by: dylan1951 <58990501+dylan1951@users.noreply.github.com>
Co-authored-by: dylan <dylanbradshaw107@hotmail.com>
Co-authored-by: Simon H <5968653+dummdidumm@users.noreply.github.com>
Co-authored-by: Simon Holthausen <simon.holthausen@vercel.com>

Author: 4 people

.changeset/clean-toes-joke.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: each block losing reactivity when items removed while promise pending