Add: Support town_production_effect and town_production_multiplier.

[PeterN]

Provides support for town production effect and multiplier properties of OpenTTD#11947

        town_production_effect:     TOWNPRODUCTION_PASSENGERS;
        town_production_multiplier: 1.0;

        town_production_effect:     TOWNPRODUCTION_MAIL;
        town_production_multiplier: 0.75;

[glx22]

Maybe we need confirmation it works from an NML user.

[PeterN]

This has been tested with improved town industries, which replaces mail with waste.

[2TallTyler]

This PR adds the new properties to ITI: 2TallTyler/improved_town_industries#104. I have not tested this NML PR.

A question: Do I need to bump the minimum required OpenTTD version for the GRF (using a custom error), or will the properties be ignored in older versions of OpenTTD?

[PeterN]

You'll need a version check. NewGRF is not self-descriptive so older versions don't know how to skip properties they don't know.

You can do this Bananas side to make life easier, then only 14.x players will be offered a new version, though of course that doesn't stop someone manually downloading and trying to load it in an older version.