feat: erase items in area

[scarf005]

Purpose of change

make it easier to erase large number of items, e.g for perf reasons.

Describe the solution

choose rectangular area, iterate through points and erase items.

Describe alternatives you've considered

make item merging easier.

Testing

2024-03-08_17-46-20.mp4

1. select debug/map/erase items in area.
2. select grid.
3. items are erased.

[chaosvolt]

If this isn't already a thing mapgen transformations can do for mission things, being able to move or delete items via JSON would be a neat improvement for the future. Think we might already have that though.