Fix menu ordering on item update

[nateinaction]

Fixes #463

This PR reorders menu items when using wp menu item update command with the --position flag

Previous behavior is described in #463

New behavior:

$ wp menu item list sample-menu --format=csv
db_id,type,title,link,position
4,custom,Alpha,https://alpha.com,1
5,custom,Beta,https://beta.com,2

$ wp menu item update 5 --position=1
Success: Menu item updated.

$ wp menu item list sample-menu --format=csv
db_id,type,title,link,position
5,custom,Beta,https://beta.com,1
4,custom,Alpha,https://alpha.com,2

$ wp menu item update 5 --position=2        
Success: Menu item updated.

$ wp menu item list sample-menu --format=csv
db_id,type,title,link,position
4,custom,Alpha,https://alpha.com,1
5,custom,Beta,https://beta.com,2

While this fix works, it utilizes reorder_menu_items() which I believe is potentially error prone in situations where there are gaps in menu item ordering. Menu ordering gaps can be introduced using the same wp menu item update command and can cause abnormal behavior when reordering position. I'm curious about the maintainer's interest in enforcing menu item ordering without allowing gaps?

Another potential bug could be the position shown when listing the menu does not match the menu_order column in the posts table. By not displaying the exact menu_order value we can cause confusion like I had when investigating this issue.

[nateinaction]

This is close but we need slightly more complex logic for reordering. Still seeing odd ordering issues on top of problems when gaps exist in menu item order.