feat: Add carriage mod

mods/carriage/FreeColMessages.properties

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+mod.carriage.Carriage
+mod.carriage.shortDescription=Adds a new carriage unit and change wagon train requirements.
+mod.carriage.license=GPL

mods/carriage/FreeColMessages_pt_BR.properties

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+mod.carriage.name=Carruagem
+mod.carriage.shortDescription=Adiciona uma nova unidade de carruagem e altera os requisitos da carroça.
+mod.carriage.license=GPL

mods/carriage/ModMessages.properties

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+model.unit.carriage.name={{plural:%number%|one=Carriage|other=Carriages|default=Carriage}}
+model.unit.carriage.description=The Carriage is a land vehicle used to transport people.

mods/carriage/ModMessages_pt_BR.properties

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+model.unit.carriage.name={{plural:%number%|one=Carruagem|other=Carruagens|default=Carruagem}}
+model.unit.carriage.description=A Carruagem é um veículo terrestre usado para transportar pessoas.

mods/carriage/README.md

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+# Carriage
+
+This mod adds a carriage unit to the game costing 100 hammers and 20 horses. It also adds 20 horses as requirement for common wagon trains.
+
+The carriage was intended to carry only units, but the city window does not open when the unit does not have the `carryGoods` ability. It will possibly need a fix in source code for `carryUnits` ability to work as expected.
+
+![Carriage Unit](./resources/carriage.size2.png)

mods/carriage/mod.xml

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+<mod id="carriage" />

mods/carriage/resources.properties

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+image.unit.model.unit.carriage=resources/carriage.png

mods/carriage/specification.xml

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+<?xml version="1.0" encoding="UTF-8"?>
+<freecol-specification id="carriage">
+  <unit-types>
+    <unit-type id="model.unit.wagonTrain" extends="wagon"
+               offence="0" defence="1" movement="6"
+               score-value="1" spaceTaken="2" space="2">
+      <required-goods id="model.goods.hammers"
+                      value="40"/>
+      <required-goods id="model.goods.horses"
+                      value="20"/>
+      <limit id="model.limit.wagonTrains" operator="lt">
+        <left-hand-side operand-type="units" scope-level="player" />
+        <right-hand-side operand-type="settlements" scope-level="player" />
+      </limit>
+      <ability id="model.ability.carryGoods"
+               value="true"/>
+      <ability id="model.ability.canBeCaptured"
+               value="true"/>
+    </unit-type>
+    <unit-type id="model.unit.carriage" extends="wagon"
+               offence="0" defence="1" movement="6"
+               score-value="1" spaceTaken="2" space="2">
+      <required-goods id="model.goods.hammers"
+                      value="100"/>
+      <required-goods id="model.goods.horses"
+                      value="20"/>
+      <ability id="model.ability.carryUnits"
+               value="true"/>
+      <ability id="model.ability.carryGoods"
+               value="true"/>
+      <ability id="model.ability.canBeCaptured"
+               value="true"/>
+    </unit-type>
+  </unit-types>
+</freecol-specification>