Add `light_area` as directional lighting source with coordinates.

[astrochili]

The lighting area could seamlessly update environmental properties such as ambient, sunlight, and fog colors.

In TrenchBroom it could be a yellow flag texture.

[astrochili]

• Another idea is that the coordinates could be the size of a box with a point of light as the center.
• The direction isn't required, it can be like a point of light, but with the area of a rectangle instead of a sphere.