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UI: Reorder mesh merge operator types for consistent shortcuts

In vertex selection mode, the "At First" and "At Last" options are
contextually added to the beginning of the list of merge operations.
This means the automatic 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.

Fixes T84335

Differential Revision: https://developer.blender.org/D10008