Ability to read spawner current chances

[randomdude]

Right now, there is no way to manually create biter units based on the current evolution weights for a given spawner.

In biter battles units are manually spawned when the units around a nest are not enough to match the desired group size.

Right now it does a heavy find_entities_filtered filtered by type=unit and calculates the distribution by hand. There are ways to improve it by using other methods like LuaForce.get_entity_count, or reading the prototype distributions and maybe interpolating (linearly?) between them to get the actual distribution, but maybe it would be easier if those values were exposed.

Or even better, add a function to spawners to create_unit, similar to create_entity, that handles internally the chance of each unit type and position of the newly created unit, so none of this needs to be done by the script or exposed