Add function to remove unit from squad

[ong-yinggao98]

Originally intended to be used in a now-cancelled Lua script feature, but figured it may be useful for those dabbling in military matters.

[myk002]

Could you expose this function to Lua (in LuaAPI.cpp) and add documentation in Lua API.rst?

Also, needs changelog under the API and Lua (after being exposed to Lua) sections

[myk002]

LGTM -- should I remove the "draft" PR status?

[ong-yinggao98]

LGTM -- should I remove the "draft" PR status?

Sure, thank you!

Do you think that the function should return a value? If it returns false at any point after changing game state it would likely be a bug that leaves game state inconsistent, so maybe I should just have it be void and expect that it works fine?

[myk002]

Do you think that the function should return a value?

That's a good point. I think that it should return false if the unit is not in a squad or other preconditions fail. I think it should return true if it has updated all the state that is feasible to update. If some dependent data structure cannot be found, I think it can skip it and still return true.