Add a new optional cliff_removal_probability/max_distance property to resources

[braxbro]

In my mod, ClaustOrephobic, I modify the prototypes of basically every resource so that the whole world is covered in ore that blocks construction of non-drill/pumpjack buildings. Currently I have an issue where cliffs do not function, because cliff generation, like tree generation, will only place cliffs in places that aren't occupied by things.

Hence my suggestion. I'd like for resources (and anything else that has tree_removal_probability/max_distance) to have a corresponding property for cliffs, either permitting cliffs to be placed on top of them or making cliffs delete the obstructions instead, because I'd really rather not have to re-implement cliff generation as a noise expression so I can override ore probability to 0 in affected areas.

Thanks.

[Rseding91]

I've added ResourceEntityPrototype::cliff_removal_probability for the next 1.1 release.