In vertex selection mode, the "At First" and "At Last" options are
added to the beginning of the list of merge operations. This means
the automatically added shortcuts in the merge menu are different,
making learning the shortcuts much more difficult than necessary.

This patch reorders the items so that "At First" and "At Last" are
added after the first three options so the shortcuts are always
consistent.

There is some risk of breaking muscle memory, but I believe the
improvement is worth it, since the reordering basically breaks
the shortcut idea here.

Fixes T84335