Hi, while I haven't played Eastward, looking at a few trailers I don't think the game uses Normal Maps. You may be able to achieve a similar look using just 2D lighting, there are plenty of tutorials for that, but one I used in the past was this tutorial by Brackeys for Unity. It essentially boils down to making the scene dark by default and only adding color where there is light, depending on the intensity and color of the light itself.
Normal Maps are not required for lighting in 2D, but if you want your textures to be affected by light based on its direction, you need normal maps. There is another Unity tutorial by Brackeys that goes into normal maps.
If you do want to use Normal Maps, you can try out my Normal Shading extension. It adds basic lighting to Aseprite and allows you to get an idea of what your texture will look like with a normal map applied :)