I've been searching for vertical editing sequential numbering, I've found: https://www.emeditor.com/text-editor-features/coding/multiple-selection-edit...
On vertical selection, a menu option allows for sequential numbering to be inserted. This would be a welcome addition to Geany. It probably would be best/proper to add it to Scintilla but it could probably be added to Geany by cycling over the vertical selections or prior to paste. 