Overall a neat concept, although it wasn't clear what made this game special until the flashlight mechanic was fully explained. While this flashlight mechanic has been used in other games, I agree that it is the main selling point for the game, and the lack of ability to deal with enemies in actual combat will add to the tension (while also reducing your scope as a developer, yay!)
Definitely look into Unity's 2D light system, however just be aware that that can slow down a bit on WebGL builds without the right settings / with too many little lights. It will be key to get this environment looking the way you want it early, and for you to work out the collider for the flashlight (this may need to be raycasts, or simpler is just a collider)
Looking forward to this one!