[2019] The current map editor is terrible

If you want to have an infinite supply of an item, place down Inifinity Chest (it's in the "question mark" category in Entity Editor) and set its filter to the item you want (artillery shell) and set how much should be kept in the chest inventory. As you take out items out of it, the chest will respawn new items to fullfill the filter settings. So then you can Ctrl + click the chest and top up supplies in your inventory instead of having to pickup items from ground.

Also, did you know you don't need to save scenario and create new map, you can just type /editor in the chat/console and it will switch you to character mode? Type /editor again to switch back to editor mode. This also works in your existing saves (will disable achievements). Or you don't need to bother with switching to character mode, you can setup inserter to feed artillery turret from infinity chest and unpause time in the editor mode.

We make the editor for ourselves to develop the game. During development of 0.17 we started to work on the new campaign so we rewamped the editor to fit our current needs. Big problem of the original one was that its logic was completely separate from normal game logic, and when new things were added to the game, we often didn't add them to the editor until somebody needed them, so it was in many aspects inferior tool to actual freeplay with cheats. Problem of the new editor is that it's completely undocumented and not very used element of the game, so there is not good knowledge in the community about how to use it.