Support showing the insertion point after the last element of a block list.

[youknowriad]

After #27860 the insertion point (in between insierter) only shows up between two blocks due to a complex positioning logic (center it properly between blocks both vertically and horizontally), this had the side effect of removing the possibility to show at the end of the block list (for instance by selecting the last block, opening the inserter and hovering a block type).

Ideally, we could have a fallback in the calculations to show it right after the last block.