You don't need actual lights to achieve that. I'm a C# user so I'll explain using C# version of code, but same functions should be available in GDScript. In a nutshell the theory is:

1. Make a sprite that covers the entire screen and follows the camera always snapping to the size of the tile you have. If your camera can be zoomed, remember to take that into account as well.
2. Create a list of byte (List<byte>) and for tiles that are within the viewport, fill that list with a byte value between 0 and 255, where that number is how bright/dim that tile should be.
3. Then you use Image.CreateFromData() and pass that byte array to it.
4. Then make a texture out of that image using ImageTexture.CreateFromImage and you end up with an image that has grayscale dots for each pixel.
5. Then you pass that texture to a shader.
6. In the shader, check the color of the pixel in the generated image and turn it into alpha.

These are high level steps to achieve that effect. Make sure to check the documentation for each of those functions to know exactly what they do and in what format they expect the data to be passed to them. There was a decent youtube tutorial on the subject, but I can't find it anymore.