Implement `attribute_8` (survivalist)

[j-wielg]

Implement the survivalist attribute, which removes the movement penalty caused by moving through a forest tile. This can be achieved by overriding the calc_move_cost method.

See here for more info on attributes.

Requirements:

• Code should be located at Attributes/Troops/Logic/attribute_8.gd
• Code should contain a comment labeling it as the survivalist attribute

[AGaylord6]

I submitted a pull request trying this, let me know how it looks!