fix(core): fix unnesting blocks with siblings (BLO-1017) (#2601)

* fix(core): add custom liftItem to fix unnesting blocks with siblings (BLO-1017)

ProseMirror's upstream liftListItem doesn't work with BlockNote's strict
blockContainer schema (blockContent blockGroup?), producing RangeError
when unnesting blocks that have siblings after them. This adds a minimal
adaptation of liftToOuterList (mirroring the existing sinkItem adaptation)
that correctly handles merging siblings into existing child blockGroups.

Fixes BLO-835, BLO-899, BLO-953, BLO-844, BLO-847.

* chore: address review feedback (const, unused import)

* test: add test case for nesting block into sibling with existing children

* chore: fix lint

Author: YousefED