Bug 1785801 - Make `RangeUpdater::SelAdjJoinNodes` take the ex-offset of right...

Bug 1785801 - Make `RangeUpdater::SelAdjJoinNodes` take the ex-offset of right node. r=m_kato, a=RyanVM

In bug 1739524, I misunderstood the meaning of `aOffset` of `SelAdjJoinNodes`.

After joining 2 nodes, and a point points right node which will have ex-left
node content, the point needs to point ex-start of the right node to keep
next insertion point as-is.  Therefore, it's not useful with new join nodes
direction, it needs to know the ex-offset of the right node.

Differential Revision: https://phabricator.services.mozilla.com/D155438