Add a confirmation prompt to /clearobjects and /clearobjects quick

It already happened to me a lot of times that I accidentally typed /clearobjects instead of //clearobjects (command from World Edit to remove objects in a marked region) and had all objects on the server deleted.

I suggest to add a prompt (chat message) that asks the user if they really want to proceed with this action and that it might take a long time (in full mode).

If server owners run those commands by accident (for whatever reason), A LOT of progress could get lost depending on the game type and mods).

EDIT: Removed formspec as prompt option since a chat message is more than enough.