Add an option to edit menu.

Currently, [```addmenuitem.php```](https://github.com/arnav-deep/FoodShala/blob/master/addmenuitem.php) only provides restaurant admins the feature to add new items on their menus.

Add a new feature in [```addmenuitem.php```](https://github.com/arnav-deep/FoodShala/blob/master/addmenuitem.php) so that restaurant admins can update their old menu, like modification of price, deletion of some item, change in dish name, etc.

In this Edit feature, any data of a menu item can be updated, or the item completely deleted.

Start with addmenuitem.php and trace how restaurant admins create and persist menu items. Compare the existing menu data flow with the requested edit and delete actions; done means admins can change any menu item data or remove an item and the updated menu is reflected afterward.

Written by the indexing model from the issue text.