Add civ2civ3's tech_cost_style to engine
civ2civ3 tech costs are set explicitly, but README.civ2civ3 says the rule is:
"The cost of each Tech is equal to the number of steps needed to get the tech (Tech Cost = Base Cost * Number of Parents). It is a linear formula more similar to civ2 [...]"
If this is to become the default ruleset, we should probably have this tech_cost_style in the engine (plus it would make civ2civ3 maintenance easier). Perhaps call it "Linear" (since other f(parents) costs are not linear)?